Merge branch 'db/checkout'

* db/checkout: (21 commits)
  checkout: error out when index is unmerged even with -m
  checkout: show progress when checkout takes long time while switching branches
  Add merge-subtree back
  checkout: updates to tracking report
  builtin-checkout.c: Remove unused prefix arguments in switch_branches path
  checkout: work from a subdirectory
  checkout: tone down the "forked status" diagnostic messages
  Clean up reporting differences on branch switch
  builtin-checkout.c: fix possible usage segfault
  checkout: notice when the switched branch is behind or forked
  Build in checkout
  Move code to clean up after a branch change to branch.c
  Library function to check for unmerged index entries
  Use diff -u instead of diff in t7201
  Move create_branch into a library file
  Build-in merge-recursive
  Add "skip_unmerged" option to unpack_trees.
  Discard "deleted" cache entries after using them to update the working tree
  Send unpack-trees debugging output to stderr
  Add flag to make unpack_trees() not print errors.
  ...

Conflicts:

	Makefile

20 files changed, 1002 insertions, 271 deletions

diff --git a/Makefile b/Makefile
index 8d9f11e75..a5b6eebf1 100644
--- a/Makefile
+++ b/Makefile
@@ -226,7 +226,7 @@ BASIC_CFLAGS =
 BASIC_LDFLAGS =
 
 SCRIPT_SH = \
-	git-bisect.sh git-checkout.sh \
+	git-bisect.sh \
 	git-clone.sh \
 	git-merge-one-file.sh git-mergetool.sh git-parse-remote.sh \
 	git-pull.sh git-rebase.sh git-rebase--interactive.sh \
@@ -265,23 +265,23 @@ PROGRAMS = \
 	git-upload-pack$X \
 	git-pack-redundant$X git-var$X \
 	git-merge-tree$X git-imap-send$X \
-	git-merge-recursive$X \
 	$(EXTRA_PROGRAMS)
 
 # Empty...
 EXTRA_PROGRAMS =
 
+# List built-in command $C whose implementation cmd_$C() is not in
+# builtin-$C.o but is linked in as part of some other command.
 BUILT_INS = \
 	git-format-patch$X git-show$X git-whatchanged$X git-cherry$X \
 	git-get-tar-commit-id$X git-init$X git-repo-config$X \
 	git-fsck-objects$X git-cherry-pick$X git-peek-remote$X git-status$X \
+	git-merge-subtree$X \
 	$(patsubst builtin-%.o,git-%$X,$(BUILTIN_OBJS))
 
 # what 'all' will build and 'install' will install, in gitexecdir
 ALL_PROGRAMS = $(PROGRAMS) $(SCRIPTS)
 
-ALL_PROGRAMS += git-merge-subtree$X
-
 # what 'all' will build but not install in gitexecdir
 OTHER_PROGRAMS = git$X gitweb/gitweb.cgi
 
@@ -327,7 +327,7 @@ LIB_OBJS = \
 	alloc.o merge-file.o path-list.o help.o unpack-trees.o $(DIFF_OBJS) \
 	color.o wt-status.o archive-zip.o archive-tar.o shallow.o utf8.o \
 	convert.o attr.o decorate.o progress.o mailmap.o symlinks.o remote.o \
-	transport.o bundle.o walker.o parse-options.o ws.o archive.o \
+	transport.o bundle.o walker.o parse-options.o ws.o archive.o branch.o \
 	alias.o
 
 BUILTIN_OBJS = \
@@ -340,6 +340,7 @@ BUILTIN_OBJS = \
 	builtin-bundle.o \
 	builtin-cat-file.o \
 	builtin-check-attr.o \
+	builtin-checkout.o \
 	builtin-checkout-index.o \
 	builtin-check-ref-format.o \
 	builtin-clean.o \
@@ -370,6 +371,7 @@ BUILTIN_OBJS = \
 	builtin-merge-base.o \
 	builtin-merge-file.o \
 	builtin-merge-ours.o \
+	builtin-merge-recursive.o \
 	builtin-mv.o \
 	builtin-name-rev.o \
 	builtin-pack-objects.o \
@@ -840,9 +842,6 @@ help.o: help.c common-cmds.h GIT-CFLAGS
 		'-DGIT_MAN_PATH="$(mandir_SQ)"' \
 		'-DGIT_INFO_PATH="$(infodir_SQ)"' $<
 
-git-merge-subtree$X: git-merge-recursive$X
-	$(QUIET_BUILT_IN)$(RM) $@ && ln git-merge-recursive$X $@
-
 $(BUILT_INS): git$X
 	$(QUIET_BUILT_IN)$(RM) $@ && ln git$X $@
 
diff --git a/branch.c b/branch.c
new file mode 100644
index 000000000..1fc878889
--- /dev/null
+++ b/branch.c
@@ -0,0 +1,148 @@
+#include "cache.h"
+#include "branch.h"
+#include "refs.h"
+#include "remote.h"
+#include "commit.h"
+
+struct tracking {
+	struct refspec spec;
+	char *src;
+	const char *remote;
+	int matches;
+};
+
+static int find_tracked_branch(struct remote *remote, void *priv)
+{
+	struct tracking *tracking = priv;
+
+	if (!remote_find_tracking(remote, &tracking->spec)) {
+		if (++tracking->matches == 1) {
+			tracking->src = tracking->spec.src;
+			tracking->remote = remote->name;
+		} else {
+			free(tracking->spec.src);
+			if (tracking->src) {
+				free(tracking->src);
+				tracking->src = NULL;
+			}
+		}
+		tracking->spec.src = NULL;
+	}
+
+	return 0;
+}
+
+/*
+ * This is called when new_ref is branched off of orig_ref, and tries
+ * to infer the settings for branch.<new_ref>.{remote,merge} from the
+ * config.
+ */
+static int setup_tracking(const char *new_ref, const char *orig_ref)
+{
+	char key[1024];
+	struct tracking tracking;
+
+	if (strlen(new_ref) > 1024 - 7 - 7 - 1)
+		return error("Tracking not set up: name too long: %s",
+				new_ref);
+
+	memset(&tracking, 0, sizeof(tracking));
+	tracking.spec.dst = (char *)orig_ref;
+	if (for_each_remote(find_tracked_branch, &tracking) ||
+			!tracking.matches)
+		return 1;
+
+	if (tracking.matches > 1)
+		return error("Not tracking: ambiguous information for ref %s",
+				orig_ref);
+
+	if (tracking.matches == 1) {
+		sprintf(key, "branch.%s.remote", new_ref);
+		git_config_set(key, tracking.remote ?  tracking.remote : ".");
+		sprintf(key, "branch.%s.merge", new_ref);
+		git_config_set(key, tracking.src);
+		free(tracking.src);
+		printf("Branch %s set up to track remote branch %s.\n",
+			       new_ref, orig_ref);
+	}
+
+	return 0;
+}
+
+void create_branch(const char *head,
+		   const char *name, const char *start_name,
+		   int force, int reflog, int track)
+{
+	struct ref_lock *lock;
+	struct commit *commit;
+	unsigned char sha1[20];
+	char *real_ref, ref[PATH_MAX], msg[PATH_MAX + 20];
+	int forcing = 0;
+
+	snprintf(ref, sizeof ref, "refs/heads/%s", name);
+	if (check_ref_format(ref))
+		die("'%s' is not a valid branch name.", name);
+
+	if (resolve_ref(ref, sha1, 1, NULL)) {
+		if (!force)
+			die("A branch named '%s' already exists.", name);
+		else if (!is_bare_repository() && !strcmp(head, name))
+			die("Cannot force update the current branch.");
+		forcing = 1;
+	}
+
+	real_ref = NULL;
+	if (get_sha1(start_name, sha1))
+		die("Not a valid object name: '%s'.", start_name);
+
+	switch (dwim_ref(start_name, strlen(start_name), sha1, &real_ref)) {
+	case 0:
+		/* Not branching from any existing branch */
+		real_ref = NULL;
+		break;
+	case 1:
+		/* Unique completion -- good */
+		break;
+	default:
+		die("Ambiguous object name: '%s'.", start_name);
+		break;
+	}
+
+	if ((commit = lookup_commit_reference(sha1)) == NULL)
+		die("Not a valid branch point: '%s'.", start_name);
+	hashcpy(sha1, commit->object.sha1);
+
+	lock = lock_any_ref_for_update(ref, NULL, 0);
+	if (!lock)
+		die("Failed to lock ref for update: %s.", strerror(errno));
+
+	if (reflog)
+		log_all_ref_updates = 1;
+
+	if (forcing)
+		snprintf(msg, sizeof msg, "branch: Reset from %s",
+			 start_name);
+	else
+		snprintf(msg, sizeof msg, "branch: Created from %s",
+			 start_name);
+
+	/* When branching off a remote branch, set up so that git-pull
+	   automatically merges from there.  So far, this is only done for
+	   remotes registered via .git/config.  */
+	if (real_ref && track)
+		setup_tracking(name, real_ref);
+
+	if (write_ref_sha1(lock, sha1, msg) < 0)
+		die("Failed to write ref: %s.", strerror(errno));
+
+	if (real_ref)
+		free(real_ref);
+}
+
+void remove_branch_state(void)
+{
+	unlink(git_path("MERGE_HEAD"));
+	unlink(git_path("rr-cache/MERGE_RR"));
+	unlink(git_path("MERGE_MSG"));
+	unlink(git_path("SQUASH_MSG"));
+}
diff --git a/branch.h b/branch.h
new file mode 100644
index 000000000..d30abe036
--- /dev/null
+++ b/branch.h
@@ -0,0 +1,24 @@
+#ifndef BRANCH_H
+#define BRANCH_H
+
+/* Functions for acting on the information about branches. */
+
+/*
+ * Creates a new branch, where head is the branch currently checked
+ * out, name is the new branch name, start_name is the name of the
+ * existing branch that the new branch should start from, force
+ * enables overwriting an existing (non-head) branch, reflog creates a
+ * reflog for the branch, and track causes the new branch to be
+ * configured to merge the remote branch that start_name is a tracking
+ * branch for (if any).
+ */
+void create_branch(const char *head, const char *name, const char *start_name,
+		   int force, int reflog, int track);
+
+/*
+ * Remove information about the state of working on the current
+ * branch. (E.g., MERGE_HEAD)
+ */
+void remove_branch_state(void);
+
+#endif
diff --git a/builtin-branch.c b/builtin-branch.c
index 9edf2eb81..7e991030c 100644
--- a/builtin-branch.c
+++ b/builtin-branch.c
@@ -12,6 +12,7 @@
 #include "builtin.h"
 #include "remote.h"
 #include "parse-options.h"
+#include "branch.h"
 
 static const char * const builtin_branch_usage[] = {
 	"git-branch [options] [-r | -a]",
@@ -359,141 +360,6 @@ static void print_ref_list(int kinds, int detached, int verbose, int abbrev, str
 	free_ref_list(&ref_list);
 }
 
-struct tracking {
-	struct refspec spec;
-	char *src;
-	const char *remote;
-	int matches;
-};
-
-static int find_tracked_branch(struct remote *remote, void *priv)
-{
-	struct tracking *tracking = priv;
-
-	if (!remote_find_tracking(remote, &tracking->spec)) {
-		if (++tracking->matches == 1) {
-			tracking->src = tracking->spec.src;
-			tracking->remote = remote->name;
-		} else {
-			free(tracking->spec.src);
-			if (tracking->src) {
-				free(tracking->src);
-				tracking->src = NULL;
-			}
-		}
-		tracking->spec.src = NULL;
-	}
-
-	return 0;
-}
-
-
-/*
- * This is called when new_ref is branched off of orig_ref, and tries
- * to infer the settings for branch.<new_ref>.{remote,merge} from the
- * config.
- */
-static int setup_tracking(const char *new_ref, const char *orig_ref)
-{
-	char key[1024];
-	struct tracking tracking;
-
-	if (strlen(new_ref) > 1024 - 7 - 7 - 1)
-		return error("Tracking not set up: name too long: %s",
-				new_ref);
-
-	memset(&tracking, 0, sizeof(tracking));
-	tracking.spec.dst = (char *)orig_ref;
-	if (for_each_remote(find_tracked_branch, &tracking) ||
-			!tracking.matches)
-		return 1;
-
-	if (tracking.matches > 1)
-		return error("Not tracking: ambiguous information for ref %s",
-				orig_ref);
-
-	if (tracking.matches == 1) {
-		sprintf(key, "branch.%s.remote", new_ref);
-		git_config_set(key, tracking.remote ?  tracking.remote : ".");
-		sprintf(key, "branch.%s.merge", new_ref);
-		git_config_set(key, tracking.src);
-		free(tracking.src);
-		printf("Branch %s set up to track remote branch %s.\n",
-			       new_ref, orig_ref);
-	}
-
-	return 0;
-}
-
-static void create_branch(const char *name, const char *start_name,
-			  int force, int reflog, int track)
-{
-	struct ref_lock *lock;
-	struct commit *commit;
-	unsigned char sha1[20];
-	char *real_ref, ref[PATH_MAX], msg[PATH_MAX + 20];
-	int forcing = 0;
-
-	snprintf(ref, sizeof ref, "refs/heads/%s", name);
-	if (check_ref_format(ref))
-		die("'%s' is not a valid branch name.", name);
-
-	if (resolve_ref(ref, sha1, 1, NULL)) {
-		if (!force)
-			die("A branch named '%s' already exists.", name);
-		else if (!is_bare_repository() && !strcmp(head, name))
-			die("Cannot force update the current branch.");
-		forcing = 1;
-	}
-
-	real_ref = NULL;
-	if (get_sha1(start_name, sha1))
-		die("Not a valid object name: '%s'.", start_name);
-
-	switch (dwim_ref(start_name, strlen(start_name), sha1, &real_ref)) {
-	case 0:
-		/* Not branching from any existing branch */
-		real_ref = NULL;
-		break;
-	case 1:
-		/* Unique completion -- good */
-		break;
-	default:
-		die("Ambiguous object name: '%s'.", start_name);
-		break;
-	}
-
-	if ((commit = lookup_commit_reference(sha1)) == NULL)
-		die("Not a valid branch point: '%s'.", start_name);
-	hashcpy(sha1, commit->object.sha1);
-
-	lock = lock_any_ref_for_update(ref, NULL, 0);
-	if (!lock)
-		die("Failed to lock ref for update: %s.", strerror(errno));
-
-	if (reflog)
-		log_all_ref_updates = 1;
-
-	if (forcing)
-		snprintf(msg, sizeof msg, "branch: Reset from %s",
-			 start_name);
-	else
-		snprintf(msg, sizeof msg, "branch: Created from %s",
-			 start_name);
-
-	/* When branching off a remote branch, set up so that git-pull
-	   automatically merges from there.  So far, this is only done for
-	   remotes registered via .git/config.  */
-	if (real_ref && track)
-		setup_tracking(name, real_ref);
-
-	if (write_ref_sha1(lock, sha1, msg) < 0)
-		die("Failed to write ref: %s.", strerror(errno));
-
-	if (real_ref)
-		free(real_ref);
-}
-
 static void rename_branch(const char *oldname, const char *newname, int force)
 {
 	char oldref[PATH_MAX], newref[PATH_MAX], logmsg[PATH_MAX*2 + 100];
@@ -618,7 +484,7 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 	else if (rename && (argc == 2))
 		rename_branch(argv[0], argv[1], rename > 1);
 	else if (argc <= 2)
-		create_branch(argv[0], (argc == 2) ? argv[1] : head,
+		create_branch(head, argv[0], (argc == 2) ? argv[1] : head,
 			      force_create, reflog, track);
 	else
 		usage_with_options(builtin_branch_usage, options);
diff --git a/builtin-checkout.c b/builtin-checkout.c
new file mode 100644
index 000000000..d5f093094
--- /dev/null
+++ b/builtin-checkout.c
@@ -0,0 +1,573 @@
+#include "cache.h"
+#include "builtin.h"
+#include "parse-options.h"
+#include "refs.h"
+#include "commit.h"
+#include "tree.h"
+#include "tree-walk.h"
+#include "unpack-trees.h"
+#include "dir.h"
+#include "run-command.h"
+#include "merge-recursive.h"
+#include "branch.h"
+#include "diff.h"
+#include "revision.h"
+#include "remote.h"
+
+static const char * const checkout_usage[] = {
+	"git checkout [options] <branch>",
+	"git checkout [options] [<branch>] -- <file>...",
+	NULL,
+};
+
+static int post_checkout_hook(struct commit *old, struct commit *new,
+			      int changed)
+{
+	struct child_process proc;
+	const char *name = git_path("hooks/post-checkout");
+	const char *argv[5];
+
+	if (access(name, X_OK) < 0)
+		return 0;
+
+	memset(&proc, 0, sizeof(proc));
+	argv[0] = name;
+	argv[1] = xstrdup(sha1_to_hex(old->object.sha1));
+	argv[2] = xstrdup(sha1_to_hex(new->object.sha1));
+	argv[3] = changed ? "1" : "0";
+	argv[4] = NULL;
+	proc.argv = argv;
+	proc.no_stdin = 1;
+	proc.stdout_to_stderr = 1;
+	return run_command(&proc);
+}
+
+static int update_some(const unsigned char *sha1, const char *base, int baselen,
+		       const char *pathname, unsigned mode, int stage)
+{
+	int len;
+	struct cache_entry *ce;
+
+	if (S_ISGITLINK(mode))
+		return 0;
+
+	if (S_ISDIR(mode))
+		return READ_TREE_RECURSIVE;
+
+	len = baselen + strlen(pathname);
+	ce = xcalloc(1, cache_entry_size(len));
+	hashcpy(ce->sha1, sha1);
+	memcpy(ce->name, base, baselen);
+	memcpy(ce->name + baselen, pathname, len - baselen);
+	ce->ce_flags = create_ce_flags(len, 0);
+	ce->ce_mode = create_ce_mode(mode);
+	add_cache_entry(ce, ADD_CACHE_OK_TO_ADD | ADD_CACHE_OK_TO_REPLACE);
+	return 0;
+}
+
+static int read_tree_some(struct tree *tree, const char **pathspec)
+{
+	int newfd;
+	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
+	newfd = hold_locked_index(lock_file, 1);
+	read_cache();
+
+	read_tree_recursive(tree, "", 0, 0, pathspec, update_some);
+
+	if (write_cache(newfd, active_cache, active_nr) ||
+	    commit_locked_index(lock_file))
+		die("unable to write new index file");
+
+	/* update the index with the given tree's info
+	 * for all args, expanding wildcards, and exit
+	 * with any non-zero return code.
+	 */
+	return 0;
+}
+
+static int checkout_paths(const char **pathspec)
+{
+	int pos;
+	struct checkout state;
+	static char *ps_matched;
+	unsigned char rev[20];
+	int flag;
+	struct commit *head;
+
+	for (pos = 0; pathspec[pos]; pos++)
+		;
+	ps_matched = xcalloc(1, pos);
+
+	for (pos = 0; pos < active_nr; pos++) {
+		struct cache_entry *ce = active_cache[pos];
+		pathspec_match(pathspec, ps_matched, ce->name, 0);
+	}
+
+	if (report_path_error(ps_matched, pathspec, 0))
+		return 1;
+
+	memset(&state, 0, sizeof(state));
+	state.force = 1;
+	state.refresh_cache = 1;
+	for (pos = 0; pos < active_nr; pos++) {
+		struct cache_entry *ce = active_cache[pos];
+		if (pathspec_match(pathspec, NULL, ce->name, 0)) {
+			checkout_entry(ce, &state, NULL);
+		}
+	}
+
+	resolve_ref("HEAD", rev, 0, &flag);
+	head = lookup_commit_reference_gently(rev, 1);
+
+	return post_checkout_hook(head, head, 0);
+}
+
+static void show_local_changes(struct object *head)
+{
+	struct rev_info rev;
+	/* I think we want full paths, even if we're in a subdirectory. */
+	init_revisions(&rev, NULL);
+	rev.abbrev = 0;
+	rev.diffopt.output_format |= DIFF_FORMAT_NAME_STATUS;
+	add_pending_object(&rev, head, NULL);
+	run_diff_index(&rev, 0);
+}
+
+static void describe_detached_head(char *msg, struct commit *commit)
+{
+	struct strbuf sb;
+	strbuf_init(&sb, 0);
+	parse_commit(commit);
+	pretty_print_commit(CMIT_FMT_ONELINE, commit, &sb, 0, "", "", 0, 0);
+	fprintf(stderr, "%s %s... %s\n", msg,
+		find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV), sb.buf);
+	strbuf_release(&sb);
+}
+
+static int reset_to_new(struct tree *tree, int quiet)
+{
+	struct unpack_trees_options opts;
+	struct tree_desc tree_desc;
+	memset(&opts, 0, sizeof(opts));
+	opts.head_idx = -1;
+	opts.update = 1;
+	opts.reset = 1;
+	opts.merge = 1;
+	opts.fn = oneway_merge;
+	opts.verbose_update = !quiet;
+	parse_tree(tree);
+	init_tree_desc(&tree_desc, tree->buffer, tree->size);
+	if (unpack_trees(1, &tree_desc, &opts))
+		return 128;
+	return 0;
+}
+
+static void reset_clean_to_new(struct tree *tree, int quiet)
+{
+	struct unpack_trees_options opts;
+	struct tree_desc tree_desc;
+	memset(&opts, 0, sizeof(opts));
+	opts.head_idx = -1;
+	opts.skip_unmerged = 1;
+	opts.reset = 1;
+	opts.merge = 1;
+	opts.fn = oneway_merge;
+	opts.verbose_update = !quiet;
+	parse_tree(tree);
+	init_tree_desc(&tree_desc, tree->buffer, tree->size);
+	if (unpack_trees(1, &tree_desc, &opts))
+		exit(128);
+}
+
+struct checkout_opts {
+	int quiet;
+	int merge;
+	int force;
+
+	char *new_branch;
+	int new_branch_log;
+	int track;
+};
+
+struct branch_info {
+	const char *name; /* The short name used */
+	const char *path; /* The full name of a real branch */
+	struct commit *commit; /* The named commit */
+};
+
+static void setup_branch_path(struct branch_info *branch)
+{
+	struct strbuf buf;
+	strbuf_init(&buf, 0);
+	strbuf_addstr(&buf, "refs/heads/");
+	strbuf_addstr(&buf, branch->name);
+	branch->path = strbuf_detach(&buf, NULL);
+}
+
+static int merge_working_tree(struct checkout_opts *opts,
+			      struct branch_info *old, struct branch_info *new)
+{
+	int ret;
+	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
+	int newfd = hold_locked_index(lock_file, 1);
+	read_cache();
+
+	if (opts->force) {
+		ret = reset_to_new(new->commit->tree, opts->quiet);
+		if (ret)
+			return ret;
+	} else {
+		struct tree_desc trees[2];
+		struct tree *tree;
+		struct unpack_trees_options topts;
+		memset(&topts, 0, sizeof(topts));
+		topts.head_idx = -1;
+
+		refresh_cache(REFRESH_QUIET);
+
+		if (unmerged_cache()) {
+			error("you need to resolve your current index first");
+			return 1;
+		}
+
+		/* 2-way merge to the new branch */
+		topts.update = 1;
+		topts.merge = 1;
+		topts.gently = opts->merge;
+		topts.verbose_update = !opts->quiet;
+		topts.fn = twoway_merge;
+		topts.dir = xcalloc(1, sizeof(*topts.dir));
+		topts.dir->show_ignored = 1;
+		topts.dir->exclude_per_dir = ".gitignore";
+		tree = parse_tree_indirect(old->commit->object.sha1);
+		init_tree_desc(&trees[0], tree->buffer, tree->size);
+		tree = parse_tree_indirect(new->commit->object.sha1);
+		init_tree_desc(&trees[1], tree->buffer, tree->size);
+
+		if (unpack_trees(2, trees, &topts)) {
+			/*
+			 * Unpack couldn't do a trivial merge; either
+			 * give up or do a real merge, depending on
+			 * whether the merge flag was used.
+			 */
+			struct tree *result;
+			struct tree *work;
+			if (!opts->merge)
+				return 1;
+			parse_commit(old->commit);
+
+			/* Do more real merge */
+
+			/*
+			 * We update the index fully, then write the
+			 * tree from the index, then merge the new
+			 * branch with the current tree, with the old
+			 * branch as the base. Then we reset the index
+			 * (but not the working tree) to the new
+			 * branch, leaving the working tree as the
+			 * merged version, but skipping unmerged
+			 * entries in the index.
+			 */
+
+			add_files_to_cache(0, NULL, NULL);
+			work = write_tree_from_memory();
+
+			ret = reset_to_new(new->commit->tree, opts->quiet);
+			if (ret)
+				return ret;
+			merge_trees(new->commit->tree, work, old->commit->tree,
+				    new->name, "local", &result);
+			reset_clean_to_new(new->commit->tree, opts->quiet);
+		}
+	}
+
+	if (write_cache(newfd, active_cache, active_nr) ||
+	    commit_locked_index(lock_file))
+		die("unable to write new index file");
+
+	if (!opts->force)
+		show_local_changes(&new->commit->object);
+
+	return 0;
+}
+
+static void report_tracking(struct branch_info *new, struct checkout_opts *opts)
+{
+	/*
+	 * We have switched to a new branch; is it building on
+	 * top of another branch, and if so does that other branch
+	 * have changes we do not have yet?
+	 */
+	char *base;
+	unsigned char sha1[20];
+	struct commit *ours, *theirs;
+	char symmetric[84];
+	struct rev_info revs;
+	const char *rev_argv[10];
+	int rev_argc;
+	int num_ours, num_theirs;
+	const char *remote_msg;
+	struct branch *branch = branch_get(new->name);
+
+	/*
+	 * Nothing to report unless we are marked to build on top of
+	 * somebody else.
+	 */
+	if (!branch || !branch->merge || !branch->merge[0] || !branch->merge[0]->dst)
+		return;
+
+	/*
+	 * If what we used to build on no longer exists, there is
+	 * nothing to report.
+	 */
+	base = branch->merge[0]->dst;
+	if (!resolve_ref(base, sha1, 1, NULL))
+		return;
+
+	theirs = lookup_commit(sha1);
+	ours = new->commit;
+	if (!hashcmp(sha1, ours->object.sha1))
+		return; /* we are the same */
+
+	/* Run "rev-list --left-right ours...theirs" internally... */
+	rev_argc = 0;
+	rev_argv[rev_argc++] = NULL;
+	rev_argv[rev_argc++] = "--left-right";
+	rev_argv[rev_argc++] = symmetric;
+	rev_argv[rev_argc++] = "--";
+	rev_argv[rev_argc] = NULL;
+
+	strcpy(symmetric, sha1_to_hex(ours->object.sha1));
+	strcpy(symmetric + 40, "...");
+	strcpy(symmetric + 43, sha1_to_hex(theirs->object.sha1));
+
+	init_revisions(&revs, NULL);
+	setup_revisions(rev_argc, rev_argv, &revs, NULL);
+	prepare_revision_walk(&revs);
+
+	/* ... and count the commits on each side. */
+	num_ours = 0;
+	num_theirs = 0;
+	while (1) {
+		struct commit *c = get_revision(&revs);
+		if (!c)
+			break;
+		if (c->object.flags & SYMMETRIC_LEFT)
+			num_ours++;
+		else
+			num_theirs++;
+	}
+
+	if (!prefixcmp(base, "refs/remotes/")) {
+		remote_msg = " remote";
+		base += strlen("refs/remotes/");
+	} else {
+		remote_msg = "";
+	}
+
+	if (!num_theirs)
+		printf("Your branch is ahead of the tracked%s branch '%s' "
+		       "by %d commit%s.\n",
+		       remote_msg, base,
+		       num_ours, (num_ours == 1) ? "" : "s");
+	else if (!num_ours)
+		printf("Your branch is behind the tracked%s branch '%s' "
+		       "by %d commit%s,\n"
+		       "and can be fast-forwarded.\n",
+		       remote_msg, base,
+		       num_theirs, (num_theirs == 1) ? "" : "s");
+	else
+		printf("Your branch and the tracked%s branch '%s' "
+		       "have diverged,\nand respectively "
+		       "have %d and %d different commit(s) each.\n",
+		       remote_msg, base,
+		       num_ours, num_theirs);
+}
+
+static void update_refs_for_switch(struct checkout_opts *opts,
+				   struct branch_info *old,
+				   struct branch_info *new)
+{
+	struct strbuf msg;
+	const char *old_desc;
+	if (opts->new_branch) {
+		create_branch(old->name, opts->new_branch, new->name, 0,
+			      opts->new_branch_log, opts->track);
+		new->name = opts->new_branch;
+		setup_branch_path(new);
+	}
+
+	strbuf_init(&msg, 0);
+	old_desc = old->name;
+	if (!old_desc)
+		old_desc = sha1_to_hex(old->commit->object.sha1);
+	strbuf_addf(&msg, "checkout: moving from %s to %s",
+		    old_desc, new->name);
+
+	if (new->path) {
+		create_symref("HEAD", new->path, msg.buf);
+		if (!opts->quiet) {
+			if (old->path && !strcmp(new->path, old->path))
+				fprintf(stderr, "Already on \"%s\"\n",
+					new->name);
+			else
+				fprintf(stderr, "Switched to%s branch \"%s\"\n",
+					opts->new_branch ? " a new" : "",
+					new->name);
+		}
+	} else if (strcmp(new->name, "HEAD")) {
+		update_ref(msg.buf, "HEAD", new->commit->object.sha1, NULL,
+			   REF_NODEREF, DIE_ON_ERR);
+		if (!opts->quiet) {
+			if (old->path)
+				fprintf(stderr, "Note: moving to \"%s\" which isn't a local branch\nIf you want to create a new branch from this checkout, you may do so\n(now or later) by using -b with the checkout command again. Example:\n  git checkout -b <new_branch_name>\n", new->name);
+			describe_detached_head("HEAD is now at", new->commit);
+		}
+	}
+	remove_branch_state();
+	strbuf_release(&msg);
+	if (!opts->quiet && (new->path || !strcmp(new->name, "HEAD")))
+		report_tracking(new, opts);
+}
+
+static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
+{
+	int ret = 0;
+	struct branch_info old;
+	unsigned char rev[20];
+	int flag;
+	memset(&old, 0, sizeof(old));
+	old.path = resolve_ref("HEAD", rev, 0, &flag);
+	old.commit = lookup_commit_reference_gently(rev, 1);
+	if (!(flag & REF_ISSYMREF))
+		old.path = NULL;
+
+	if (old.path && !prefixcmp(old.path, "refs/heads/"))
+		old.name = old.path + strlen("refs/heads/");
+
+	if (!new->name) {
+		new->name = "HEAD";
+		new->commit = old.commit;
+		if (!new->commit)
+			die("You are on a branch yet to be born");
+		parse_commit(new->commit);
+	}
+
+	/*
+	 * If the new thing isn't a branch and isn't HEAD and we're
+	 * not starting a new branch, and we want messages, and we
+	 * weren't on a branch, and we're moving to a new commit,
+	 * describe the old commit.
+	 */
+	if (!new->path && strcmp(new->name, "HEAD") && !opts->new_branch &&
+	    !opts->quiet && !old.path && new->commit != old.commit)
+		describe_detached_head("Previous HEAD position was", old.commit);
+
+	if (!old.commit) {
+		if (!opts->quiet) {
+			fprintf(stderr, "warning: You appear to be on a branch yet to be born.\n");
+			fprintf(stderr, "warning: Forcing checkout of %s.\n", new->name);
+		}
+		opts->force = 1;
+	}
+
+	ret = merge_working_tree(opts, &old, new);
+	if (ret)
+		return ret;
+
+	update_refs_for_switch(opts, &old, new);
+
+	return post_checkout_hook(old.commit, new->commit, 1);
+}
+
+static int branch_track = 0;
+
+static int git_checkout_config(const char *var, const char *value)
+{
+	if (!strcmp(var, "branch.autosetupmerge"))
+		branch_track = git_config_bool(var, value);
+
+	return git_default_config(var, value);
+}
+
+int cmd_checkout(int argc, const char **argv, const char *prefix)
+{
+	struct checkout_opts opts;
+	unsigned char rev[20];
+	const char *arg;
+	struct branch_info new;
+	struct tree *source_tree = NULL;
+	struct option options[] = {
+		OPT__QUIET(&opts.quiet),
+		OPT_STRING('b', NULL, &opts.new_branch, "new branch", "branch"),
+		OPT_BOOLEAN('l', NULL, &opts.new_branch_log, "log for new branch"),
+		OPT_BOOLEAN( 0 , "track", &opts.track, "track"),
+		OPT_BOOLEAN('f', NULL, &opts.force, "force"),
+		OPT_BOOLEAN('m', NULL, &opts.merge, "merge"),
+		OPT_END(),
+	};
+
+	memset(&opts, 0, sizeof(opts));
+	memset(&new, 0, sizeof(new));
+
+	git_config(git_checkout_config);
+
+	opts.track = branch_track;
+
+	argc = parse_options(argc, argv, options, checkout_usage, 0);
+	if (argc) {
+		arg = argv[0];
+		if (get_sha1(arg, rev))
+			;
+		else if ((new.commit = lookup_commit_reference_gently(rev, 1))) {
+			new.name = arg;
+			setup_branch_path(&new);
+			if (resolve_ref(new.path, rev, 1, NULL))
+				new.commit = lookup_commit_reference(rev);
+			else
+				new.path = NULL;
+			parse_commit(new.commit);
+			source_tree = new.commit->tree;
+			argv++;
+			argc--;
+		} else if ((source_tree = parse_tree_indirect(rev))) {
+			argv++;
+			argc--;
+		}
+	}
+
+	if (argc && !strcmp(argv[0], "--")) {
+		argv++;
+		argc--;
+	}
+
+	if (!opts.new_branch && (opts.track != branch_track))
+		die("git checkout: --track and --no-track require -b");
+
+	if (opts.force && opts.merge)
+		die("git checkout: -f and -m are incompatible");
+
+	if (argc) {
+		const char **pathspec = get_pathspec(prefix, argv);
+		/* Checkout paths */
+		if (opts.new_branch || opts.force || opts.merge) {
+			if (argc == 1) {
+				die("git checkout: updating paths is incompatible with switching branches/forcing\nDid you intend to checkout '%s' which can not be resolved as commit?", argv[0]);
+			} else {
+				die("git checkout: updating paths is incompatible with switching branches/forcing");
+			}
+		}
+
+		if (source_tree)
+			read_tree_some(source_tree, pathspec);
+		else
+			read_cache();
+		return checkout_paths(pathspec);
+	}
+
+	if (new.name && !new.commit) {
+		die("Cannot switch branch to a non-commit.");
+	}
+
+	return switch_branches(&opts, &new);
+}
diff --git a/builtin-commit.c b/builtin-commit.c
index 065e1f7b7..f49c22e64 100644
--- a/builtin-commit.c
+++ b/builtin-commit.c
@@ -205,7 +205,8 @@ static void create_base_index(void)
 		die("failed to unpack HEAD tree object");
 	parse_tree(tree);
 	init_tree_desc(&t, tree->buffer, tree->size);
-	unpack_trees(1, &t, &opts);
+	if (unpack_trees(1, &t, &opts))
+		exit(128); /* We've already reported the error, finish dying */
 }
 
 static char *prepare_index(int argc, const char **argv, const char *prefix)
diff --git a/merge-recursive.c b/builtin-merge-recursive.c
index 55ef76f5a..6fe4102c0 100644
--- a/merge-recursive.c
+++ b/builtin-merge-recursive.c
@@ -7,6 +7,7 @@
 #include "cache-tree.h"
 #include "commit.h"
 #include "blob.h"
+#include "builtin.h"
 #include "tree-walk.h"
 #include "diff.h"
 #include "diffcore.h"
@@ -17,6 +18,7 @@
 #include "xdiff-interface.h"
 #include "interpolate.h"
 #include "attr.h"
+#include "merge-recursive.h"
 
 static int subtree_merge;
 
@@ -221,22 +223,11 @@ static int git_merge_trees(int index_only,
 	return rc;
 }
 
-static int unmerged_index(void)
-{
-	int i;
-	for (i = 0; i < active_nr; i++) {
-		struct cache_entry *ce = active_cache[i];
-		if (ce_stage(ce))
-			return 1;
-	}
-	return 0;
-}
-
-static struct tree *git_write_tree(void)
+struct tree *write_tree_from_memory(void)
 {
 	struct tree *result = NULL;
 
-	if (unmerged_index()) {
+	if (unmerged_cache()) {
 		int i;
 		output(0, "There are unmerged index entries:");
 		for (i = 0; i < active_nr; i++) {
@@ -1496,12 +1487,12 @@ static int process_entry(const char *path, struct stage_data *entry,
 	return clean_merge;
 }
 
-static int merge_trees(struct tree *head,
-		       struct tree *merge,
-		       struct tree *common,
-		       const char *branch1,
-		       const char *branch2,
-		       struct tree **result)
+int merge_trees(struct tree *head,
+		struct tree *merge,
+		struct tree *common,
+		const char *branch1,
+		const char *branch2,
+		struct tree **result)
 {
 	int code, clean;
 
@@ -1523,7 +1514,7 @@ static int merge_trees(struct tree *head,
 		    sha1_to_hex(head->object.sha1),
 		    sha1_to_hex(merge->object.sha1));
 
-	if (unmerged_index()) {
+	if (unmerged_cache()) {
 		struct path_list *entries, *re_head, *re_merge;
 		int i;
 		path_list_clear(&current_file_set, 1);
@@ -1553,7 +1544,7 @@ static int merge_trees(struct tree *head,
 		clean = 1;
 
 	if (index_only)
-		*result = git_write_tree();
+		*result = write_tree_from_memory();
 
 	return clean;
 }
@@ -1573,12 +1564,12 @@ static struct commit_list *reverse_commit_list(struct commit_list *list)
  * Merge the commits h1 and h2, return the resulting virtual
  * commit object and a flag indicating the cleanness of the merge.
  */
-static int merge(struct commit *h1,
-		 struct commit *h2,
-		 const char *branch1,
-		 const char *branch2,
-		 struct commit_list *ca,
-		 struct commit **result)
+int merge_recursive(struct commit *h1,
+		    struct commit *h2,
+		    const char *branch1,
+		    const char *branch2,
+		    struct commit_list *ca,
+		    struct commit **result)
 {
 	struct commit_list *iter;
 	struct commit *merged_common_ancestors;
@@ -1623,11 +1614,11 @@ static int merge(struct commit *h1,
 		 * "conflicts" were already resolved.
 		 */
 		discard_cache();
-		merge(merged_common_ancestors, iter->item,
-		      "Temporary merge branch 1",
-		      "Temporary merge branch 2",
-		      NULL,
-		      &merged_common_ancestors);
+		merge_recursive(merged_common_ancestors, iter->item,
+				"Temporary merge branch 1",
+				"Temporary merge branch 2",
+				NULL,
+				&merged_common_ancestors);
 		call_depth--;
 
 		if (!merged_common_ancestors)
@@ -1698,7 +1689,7 @@ static int merge_config(const char *var, const char *value)
 	return git_default_config(var, value);
 }
 
-int main(int argc, char *argv[])
+int cmd_merge_recursive(int argc, const char **argv, const char *prefix)
 {
 	static const char *bases[20];
 	static unsigned bases_count = 0;
@@ -1752,7 +1743,7 @@ int main(int argc, char *argv[])
 		struct commit *ancestor = get_ref(bases[i]);
 		ca = commit_list_insert(ancestor, &ca);
 	}
-	clean = merge(h1, h2, branch1, branch2, ca, &result);
+	clean = merge_recursive(h1, h2, branch1, branch2, ca, &result);
 
 	if (active_cache_changed &&
 	    (write_cache(index_fd, active_cache, active_nr) ||
diff --git a/builtin-read-tree.c b/builtin-read-tree.c
index 7bdc312e3..0138f5a91 100644
--- a/builtin-read-tree.c
+++ b/builtin-read-tree.c
@@ -269,7 +269,8 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 		parse_tree(tree);
 		init_tree_desc(t+i, tree->buffer, tree->size);
 	}
-	unpack_trees(nr_trees, t, &opts);
+	if (unpack_trees(nr_trees, t, &opts))
+		return 128;
 
 	/*
 	 * When reading only one tree (either the most basic form,
diff --git a/builtin-reset.c b/builtin-reset.c
index 7ee811f0b..af0037ec6 100644
--- a/builtin-reset.c
+++ b/builtin-reset.c
@@ -16,6 +16,7 @@
 #include "diff.h"
 #include "diffcore.h"
 #include "tree.h"
+#include "branch.h"
 
 static const char builtin_reset_usage[] =
 "git-reset [--mixed | --soft | --hard] [-q] [<commit-ish>] [ [--] <paths>...]";
@@ -44,18 +45,6 @@ static inline int is_merge(void)
 	return !access(git_path("MERGE_HEAD"), F_OK);
 }
 
-static int unmerged_files(void)
-{
-	int i;
-	read_cache();
-	for (i = 0; i < active_nr; i++) {
-		struct cache_entry *ce = active_cache[i];
-		if (ce_stage(ce))
-			return 1;
-	}
-	return 0;
-}
-
 static int reset_index_file(const unsigned char *sha1, int is_hard_reset)
 {
 	int i = 0;
@@ -250,7 +239,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 	 * at all, but requires them in a good order.  Other resets reset
 	 * the index file to the tree object we are switching to. */
 	if (reset_type == SOFT) {
-		if (is_merge() || unmerged_files())
+		if (is_merge() || read_cache() < 0 || unmerged_cache())
 			die("Cannot do a soft reset in the middle of a merge.");
 	}
 	else if (reset_index_file(sha1, (reset_type == HARD)))
@@ -282,10 +271,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 		break;
 	}
 
-	unlink(git_path("MERGE_HEAD"));
-	unlink(git_path("rr-cache/MERGE_RR"));
-	unlink(git_path("MERGE_MSG"));
-	unlink(git_path("SQUASH_MSG"));
+	remove_branch_state();
 
 	free(reflog_action);
 
diff --git a/builtin.h b/builtin.h
index 3d1628c59..674c8a141 100644
--- a/builtin.h
+++ b/builtin.h
@@ -18,6 +18,7 @@ extern int cmd_blame(int argc, const char **argv, const char *prefix);
 extern int cmd_branch(int argc, const char **argv, const char *prefix);
 extern int cmd_bundle(int argc, const char **argv, const char *prefix);
 extern int cmd_cat_file(int argc, const char **argv, const char *prefix);
+extern int cmd_checkout(int argc, const char **argv, const char *prefix);
 extern int cmd_checkout_index(int argc, const char **argv, const char *prefix);
 extern int cmd_check_attr(int argc, const char **argv, const char *prefix);
 extern int cmd_check_ref_format(int argc, const char **argv, const char *prefix);
@@ -56,6 +57,7 @@ extern int cmd_mailsplit(int argc, const char **argv, const char *prefix);
 extern int cmd_merge_base(int argc, const char **argv, const char *prefix);
 extern int cmd_merge_ours(int argc, const char **argv, const char *prefix);
 extern int cmd_merge_file(int argc, const char **argv, const char *prefix);
+extern int cmd_merge_recursive(int argc, const char **argv, const char *prefix);
 extern int cmd_mv(int argc, const char **argv, const char *prefix);
 extern int cmd_name_rev(int argc, const char **argv, const char *prefix);
 extern int cmd_pack_objects(int argc, const char **argv, const char *prefix);
diff --git a/cache.h b/cache.h
index 7769f055e..9ebe7913b 100644
--- a/cache.h
+++ b/cache.h
@@ -252,6 +252,7 @@ extern struct index_state the_index;
 #define read_cache_from(path) read_index_from(&the_index, (path))
 #define write_cache(newfd, cache, entries) write_index(&the_index, (newfd))
 #define discard_cache() discard_index(&the_index)
+#define unmerged_cache() unmerged_index(&the_index)
 #define cache_name_pos(name, namelen) index_name_pos(&the_index,(name),(namelen))
 #define add_cache_entry(ce, option) add_index_entry(&the_index, (ce), (option))
 #define remove_cache_entry_at(pos) remove_index_entry_at(&the_index, (pos))
@@ -346,6 +347,7 @@ extern int read_index(struct index_state *);
 extern int read_index_from(struct index_state *, const char *path);
 extern int write_index(struct index_state *, int newfd);
 extern int discard_index(struct index_state *);
+extern int unmerged_index(struct index_state *);
 extern int verify_path(const char *path);
 extern int index_name_exists(struct index_state *istate, const char *name, int namelen);
 extern int index_name_pos(struct index_state *, const char *name, int namelen);
diff --git a/git-checkout.sh b/contrib/examples/git-checkout.sh
index 1a7689a48..1a7689a48 100755
--- a/git-checkout.sh
+++ b/contrib/examples/git-checkout.sh
diff --git a/diff-lib.c b/diff-lib.c
index 03eaa7cef..94b150e83 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -737,7 +737,8 @@ int run_diff_index(struct rev_info *revs, int cached)
 	opts.unpack_data = revs;
 
 	init_tree_desc(&t, tree->buffer, tree->size);
-	unpack_trees(1, &t, &opts);
+	if (unpack_trees(1, &t, &opts))
+		exit(128);
 
 	diffcore_std(&revs->diffopt);
 	diff_flush(&revs->diffopt);
@@ -789,6 +790,7 @@ int do_diff_cache(const unsigned char *tree_sha1, struct diff_options *opt)
 	opts.unpack_data = &revs;
 
 	init_tree_desc(&t, tree->buffer, tree->size);
-	unpack_trees(1, &t, &opts);
+	if (unpack_trees(1, &t, &opts))
+		exit(128);
 	return 0;
 }
diff --git a/git.c b/git.c
index 8f08b1229..9cca81a60 100644
--- a/git.c
+++ b/git.c
@@ -278,6 +278,7 @@ static void handle_internal_command(int argc, const char **argv)
 		{ "branch", cmd_branch, RUN_SETUP },
 		{ "bundle", cmd_bundle },
 		{ "cat-file", cmd_cat_file, RUN_SETUP },
+		{ "checkout", cmd_checkout, RUN_SETUP | NEED_WORK_TREE },
 		{ "checkout-index", cmd_checkout_index,
 			RUN_SETUP | NEED_WORK_TREE},
 		{ "check-ref-format", cmd_check_ref_format },
@@ -321,6 +322,8 @@ static void handle_internal_command(int argc, const char **argv)
 		{ "merge-base", cmd_merge_base, RUN_SETUP },
 		{ "merge-file", cmd_merge_file },
 		{ "merge-ours", cmd_merge_ours, RUN_SETUP },
+		{ "merge-recursive", cmd_merge_recursive, RUN_SETUP | NEED_WORK_TREE },
+		{ "merge-subtree", cmd_merge_recursive, RUN_SETUP | NEED_WORK_TREE },
 		{ "mv", cmd_mv, RUN_SETUP | NEED_WORK_TREE },
 		{ "name-rev", cmd_name_rev, RUN_SETUP },
 		{ "pack-objects", cmd_pack_objects, RUN_SETUP },
diff --git a/merge-recursive.h b/merge-recursive.h
new file mode 100644
index 000000000..f37630a8a
--- /dev/null
+++ b/merge-recursive.h
@@ -0,0 +1,20 @@
+#ifndef MERGE_RECURSIVE_H
+#define MERGE_RECURSIVE_H
+
+int merge_recursive(struct commit *h1,
+		    struct commit *h2,
+		    const char *branch1,
+		    const char *branch2,
+		    struct commit_list *ancestors,
+		    struct commit **result);
+
+int merge_trees(struct tree *head,
+		struct tree *merge,
+		struct tree *common,
+		const char *branch1,
+		const char *branch2,
+		struct tree **result);
+
+struct tree *write_tree_from_memory(void);
+
+#endif
diff --git a/read-cache.c b/read-cache.c
index fee0c8073..657f0c589 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1166,6 +1166,16 @@ int discard_index(struct index_state *istate)
 	return 0;
 }
 
+int unmerged_index(struct index_state *istate)
+{
+	int i;
+	for (i = 0; i < istate->cache_nr; i++) {
+		if (ce_stage(istate->cache[i]))
+			return 1;
+	}
+	return 0;
+}
+
 #define WRITE_BUFFER_SIZE 8192
 static unsigned char write_buffer[WRITE_BUFFER_SIZE];
 static unsigned long write_buffer_len;
diff --git a/t/t6029-merge-subtree.sh b/t/t6029-merge-subtree.sh
new file mode 100755
index 000000000..3900a0508
--- /dev/null
+++ b/t/t6029-merge-subtree.sh
@@ -0,0 +1,32 @@
+#!/bin/sh
+
+test_description='subtree merge strategy'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+
+	s="1 2 3 4 5 6 7 8"
+	for i in $s; do echo $i; done >hello &&
+	git add hello &&
+	git commit -m initial &&
+	git checkout -b side &&
+	echo >>hello world &&
+	git add hello &&
+	git commit -m second &&
+	git checkout master &&
+	for i in mundo $s; do echo $i; done >hello &&
+	git add hello &&
+	git commit -m master
+
+'
+
+test_expect_success 'subtree available and works like recursive' '
+
+	git merge -s subtree side &&
+	for i in mundo $s world; do echo $i; done >expect &&
+	diff -u expect hello
+
+'
+
+test_done
diff --git a/t/t7201-co.sh b/t/t7201-co.sh
index dbf1ace29..724adef0b 100755
--- a/t/t7201-co.sh
+++ b/t/t7201-co.sh
@@ -83,13 +83,13 @@ test_expect_success "checkout with unrelated dirty tree without -m" '
 	fill 0 1 2 3 4 5 6 7 8 >same &&
 	cp same kept
 	git checkout side >messages &&
-	git diff same kept
+	diff -u same kept
 	(cat > messages.expect <<EOF
 M	same
 EOF
 ) &&
 	touch messages.expect &&
-	git diff messages.expect messages
+	diff -u messages.expect messages
 '
 
 test_expect_success "checkout -m with dirty tree" '
@@ -103,29 +103,22 @@ test_expect_success "checkout -m with dirty tree" '
 	test "$(git symbolic-ref HEAD)" = "refs/heads/side" &&
 
 	(cat >expect.messages <<EOF
-Merging side with local
-Merging:
-ab76817 Side M one, D two, A three
-virtual local
-found 1 common ancestor(s):
-7329388 Initial A one, A two
-Auto-merged one
 M	one
 EOF
 ) &&
-	git diff expect.messages messages &&
+	diff -u expect.messages messages &&
 
 	fill "M	one" "A	three" "D	two" >expect.master &&
 	git diff --name-status master >current.master &&
-	diff expect.master current.master &&
+	diff -u expect.master current.master &&
 
 	fill "M	one" >expect.side &&
 	git diff --name-status side >current.side &&
-	diff expect.side current.side &&
+	diff -u expect.side current.side &&
 
 	: >expect.index &&
 	git diff --cached >current.index &&
-	diff expect.index current.index
+	diff -u expect.index current.index
 '
 
 test_expect_success "checkout -m with dirty tree, renamed" '
@@ -143,7 +136,7 @@ test_expect_success "checkout -m with dirty tree, renamed" '
 
 	git checkout -m renamer &&
 	fill 1 3 4 5 7 8 >expect &&
-	diff expect uno &&
+	diff -u expect uno &&
 	! test -f one &&
 	git diff --cached >current &&
 	! test -s current
@@ -168,7 +161,7 @@ test_expect_success 'checkout -m with merge conflict' '
 	git diff master:one :3:uno |
 	sed -e "1,/^@@/d" -e "/^ /d" -e "s/^-/d/" -e "s/^+/a/" >current &&
 	fill d2 aT d7 aS >expect &&
-	diff current expect &&
+	diff -u current expect &&
 	git diff --cached two >current &&
 	! test -s current
 '
@@ -185,7 +178,7 @@ If you want to create a new branch from this checkout, you may do so
 HEAD is now at 7329388... Initial A one, A two
 EOF
 ) &&
-	git diff messages.expect messages &&
+	diff -u messages.expect messages &&
 	H=$(git rev-parse --verify HEAD) &&
 	M=$(git show-ref -s --verify refs/heads/master) &&
 	test "z$H" = "z$M" &&
@@ -286,4 +279,38 @@ test_expect_success 'checkout with ambiguous tag/branch names' '
 
 '
 
+test_expect_success 'switch branches while in subdirectory' '
+
+	git reset --hard &&
+	git checkout master &&
+
+	mkdir subs &&
+	(
+		cd subs &&
+		git checkout side
+	) &&
+	! test -f subs/one &&
+	rm -fr subs
+
+'
+
+test_expect_success 'checkout specific path while in subdirectory' '
+
+	git reset --hard &&
+	git checkout side &&
+	mkdir subs &&
+	>subs/bero &&
+	git add subs/bero &&
+	git commit -m "add subs/bero" &&
+
+	git checkout master &&
+	mkdir -p subs &&
+	(
+		cd subs &&
+		git checkout side -- bero
+	) &&
+	test -f subs/bero
+
+'
+
 test_done
diff --git a/unpack-trees.c b/unpack-trees.c
index 56c1ffbc1..3e448d897 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -85,6 +85,7 @@ static int unpack_trees_rec(struct tree_entry_list **posns, int len,
 		int any_dirs = 0;
 		char *cache_name;
 		int ce_stage;
+		int skip_entry = 0;
 
 		/* Find the first name in the input. */
 
@@ -122,13 +123,13 @@ static int unpack_trees_rec(struct tree_entry_list **posns, int len,
 
 #if DBRT_DEBUG > 1
 		if (first)
-			printf("index %s\n", first);
+			fprintf(stderr, "index %s\n", first);
 #endif
 		for (i = 0; i < len; i++) {
 			if (!posns[i] || posns[i] == df_conflict_list)
 				continue;
 #if DBRT_DEBUG > 1
-			printf("%d %s\n", i + 1, posns[i]->name);
+			fprintf(stderr, "%d %s\n", i + 1, posns[i]->name);
 #endif
 			if (!first || entcmp(first, firstdir,
 					     posns[i]->name,
@@ -153,6 +154,8 @@ static int unpack_trees_rec(struct tree_entry_list **posns, int len,
 			any_files = 1;
 			src[0] = active_cache[o->pos];
 			remove = o->pos;
+			if (o->skip_unmerged && ce_stage(src[0]))
+				skip_entry = 1;
 		}
 
 		for (i = 0; i < len; i++) {
@@ -181,6 +184,12 @@ static int unpack_trees_rec(struct tree_entry_list **posns, int len,
 				continue;
 			}
 
+			if (skip_entry) {
+				subposns[i] = df_conflict_list;
+				posns[i] = posns[i]->next;
+				continue;
+			}
+
 			if (!o->merge)
 				ce_stage = 0;
 			else if (i + 1 < o->head_idx)
@@ -205,23 +214,31 @@ static int unpack_trees_rec(struct tree_entry_list **posns, int len,
 			posns[i] = posns[i]->next;
 		}
 		if (any_files) {
-			if (o->merge) {
+			if (skip_entry) {
+				o->pos++;
+				while (o->pos < active_nr &&
+				       !strcmp(active_cache[o->pos]->name,
+					       src[0]->name))
+					o->pos++;
+			} else if (o->merge) {
 				int ret;
 
 #if DBRT_DEBUG > 1
-				printf("%s:\n", first);
+				fprintf(stderr, "%s:\n", first);
 				for (i = 0; i < src_size; i++) {
-					printf(" %d ", i);
+					fprintf(stderr, " %d ", i);
 					if (src[i])
-						printf("%s\n", sha1_to_hex(src[i]->sha1));
+						fprintf(stderr, "%06x %s\n", src[i]->ce_mode, sha1_to_hex(src[i]->sha1));
 					else
-						printf("\n");
+						fprintf(stderr, "\n");
 				}
 #endif
 				ret = o->fn(src, o, remove);
+				if (ret < 0)
+					return ret;
 
 #if DBRT_DEBUG > 1
-				printf("Added %d entries\n", ret);
+				fprintf(stderr, "Added %d entries\n", ret);
 #endif
 				o->pos += ret;
 			} else {
@@ -286,16 +303,16 @@ static void unlink_entry(char *name, char *last_symlink)
 }
 
 static struct checkout state;
-static void check_updates(struct cache_entry **src, int nr,
-			struct unpack_trees_options *o)
+static void check_updates(struct unpack_trees_options *o)
 {
 	unsigned cnt = 0, total = 0;
 	struct progress *progress = NULL;
 	char last_symlink[PATH_MAX];
+	int i;
 
 	if (o->update && o->verbose_update) {
-		for (total = cnt = 0; cnt < nr; cnt++) {
-			struct cache_entry *ce = src[cnt];
+		for (total = cnt = 0; cnt < active_nr; cnt++) {
+			struct cache_entry *ce = active_cache[cnt];
 			if (ce->ce_flags & (CE_UPDATE | CE_REMOVE))
 				total++;
 		}
@@ -306,14 +323,16 @@ static void check_updates(struct cache_entry **src, int nr,
 	}
 
 	*last_symlink = '\0';
-	while (nr--) {
-		struct cache_entry *ce = *src++;
+	for (i = 0; i < active_nr; i++) {
+		struct cache_entry *ce = active_cache[i];
 
 		if (ce->ce_flags & (CE_UPDATE | CE_REMOVE))
 			display_progress(progress, ++cnt);
 		if (ce->ce_flags & CE_REMOVE) {
 			if (o->update)
 				unlink_entry(ce->name, last_symlink);
+			remove_cache_entry_at(i);
+			i--;
 			continue;
 		}
 		if (ce->ce_flags & CE_UPDATE) {
@@ -354,23 +373,34 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 			posns[i] = create_tree_entry_list(t+i);
 
 		if (unpack_trees_rec(posns, len, o->prefix ? o->prefix : "",
-				     o, &df_conflict_list))
+				     o, &df_conflict_list)) {
+			if (o->gently) {
+				discard_cache();
+				read_cache();
+			}
 			return -1;
+		}
 	}
 
-	if (o->trivial_merges_only && o->nontrivial_merge)
-		die("Merge requires file-level merging");
+	if (o->trivial_merges_only && o->nontrivial_merge) {
+		if (o->gently) {
+			discard_cache();
+			read_cache();
+		}
+		return o->gently ? -1 :
+			error("Merge requires file-level merging");
+	}
 
-	check_updates(active_cache, active_nr, o);
+	check_updates(o);
 	return 0;
 }
 
 /* Here come the merge functions */
 
-static void reject_merge(struct cache_entry *ce)
+static int reject_merge(struct cache_entry *ce)
 {
-	die("Entry '%s' would be overwritten by merge. Cannot merge.",
-	    ce->name);
+	return error("Entry '%s' would be overwritten by merge. Cannot merge.",
+		     ce->name);
 }
 
 static int same(struct cache_entry *a, struct cache_entry *b)
@@ -388,18 +418,18 @@ static int same(struct cache_entry *a, struct cache_entry *b)
  * When a CE gets turned into an unmerged entry, we
  * want it to be up-to-date
  */
-static void verify_uptodate(struct cache_entry *ce,
+static int verify_uptodate(struct cache_entry *ce,
 		struct unpack_trees_options *o)
 {
 	struct stat st;
 
 	if (o->index_only || o->reset)
-		return;
+		return 0;
 
 	if (!lstat(ce->name, &st)) {
 		unsigned changed = ce_match_stat(ce, &st, CE_MATCH_IGNORE_VALID);
 		if (!changed)
-			return;
+			return 0;
 		/*
 		 * NEEDSWORK: the current default policy is to allow
 		 * submodule to be out of sync wrt the supermodule
@@ -408,12 +438,13 @@ static void verify_uptodate(struct cache_entry *ce,
 		 * checked out.
 		 */
 		if (S_ISGITLINK(ce->ce_mode))
-			return;
+			return 0;
 		errno = 0;
 	}
 	if (errno == ENOENT)
-		return;
-	die("Entry '%s' not uptodate. Cannot merge.", ce->name);
+		return 0;
+	return o->gently ? -1 :
+		error("Entry '%s' not uptodate. Cannot merge.", ce->name);
 }
 
 static void invalidate_ce_path(struct cache_entry *ce)
@@ -479,7 +510,8 @@ static int verify_clean_subdirectory(struct cache_entry *ce, const char *action,
 		 * ce->name is an entry in the subdirectory.
 		 */
 		if (!ce_stage(ce)) {
-			verify_uptodate(ce, o);
+			if (verify_uptodate(ce, o))
+				return -1;
 			ce->ce_flags |= CE_REMOVE;
 		}
 		cnt++;
@@ -498,8 +530,9 @@ static int verify_clean_subdirectory(struct cache_entry *ce, const char *action,
 		d.exclude_per_dir = o->dir->exclude_per_dir;
 	i = read_directory(&d, ce->name, pathbuf, namelen+1, NULL);
 	if (i)
-		die("Updating '%s' would lose untracked files in it",
-		    ce->name);
+		return o->gently ? -1 :
+			error("Updating '%s' would lose untracked files in it",
+			      ce->name);
 	free(pathbuf);
 	return cnt;
 }
@@ -508,16 +541,16 @@ static int verify_clean_subdirectory(struct cache_entry *ce, const char *action,
  * We do not want to remove or overwrite a working tree file that
  * is not tracked, unless it is ignored.
  */
-static void verify_absent(struct cache_entry *ce, const char *action,
-		struct unpack_trees_options *o)
+static int verify_absent(struct cache_entry *ce, const char *action,
+			 struct unpack_trees_options *o)
 {
 	struct stat st;
 
 	if (o->index_only || o->reset || !o->update)
-		return;
+		return 0;
 
 	if (has_symlink_leading_path(ce->name, NULL))
-		return;
+		return 0;
 
 	if (!lstat(ce->name, &st)) {
 		int cnt;
@@ -528,7 +561,7 @@ static void verify_absent(struct cache_entry *ce, const char *action,
 			 * ce->name is explicitly excluded, so it is Ok to
 			 * overwrite it.
 			 */
-			return;
+			return 0;
 		if (S_ISDIR(st.st_mode)) {
 			/*
 			 * We are checking out path "foo" and
@@ -557,7 +590,7 @@ static void verify_absent(struct cache_entry *ce, const char *action,
 			 * deleted entries here.
 			 */
 			o->pos += cnt;
-			return;
+			return 0;
 		}
 
 		/*
@@ -569,12 +602,14 @@ static void verify_absent(struct cache_entry *ce, const char *action,
 		if (0 <= cnt) {
 			struct cache_entry *ce = active_cache[cnt];
 			if (ce->ce_flags & CE_REMOVE)
-				return;
+				return 0;
 		}
 
-		die("Untracked working tree file '%s' "
-		    "would be %s by merge.", ce->name, action);
+		return o->gently ? -1 :
+			error("Untracked working tree file '%s' "
+			      "would be %s by merge.", ce->name, action);
 	}
+	return 0;
 }
 
 static int merged_entry(struct cache_entry *merge, struct cache_entry *old,
@@ -592,12 +627,14 @@ static int merged_entry(struct cache_entry *merge, struct cache_entry *old,
 		if (same(old, merge)) {
 			copy_cache_entry(merge, old);
 		} else {
-			verify_uptodate(old, o);
+			if (verify_uptodate(old, o))
+				return -1;
 			invalidate_ce_path(old);
 		}
 	}
 	else {
-		verify_absent(merge, "overwritten", o);
+		if (verify_absent(merge, "overwritten", o))
+			return -1;
 		invalidate_ce_path(merge);
 	}
 
@@ -609,10 +646,12 @@ static int merged_entry(struct cache_entry *merge, struct cache_entry *old,
 static int deleted_entry(struct cache_entry *ce, struct cache_entry *old,
 		struct unpack_trees_options *o)
 {
-	if (old)
-		verify_uptodate(old, o);
-	else
-		verify_absent(ce, "removed", o);
+	if (old) {
+		if (verify_uptodate(old, o))
+			return -1;
+	} else
+		if (verify_absent(ce, "removed", o))
+			return -1;
 	ce->ce_flags |= CE_REMOVE;
 	add_cache_entry(ce, ADD_CACHE_OK_TO_ADD|ADD_CACHE_OK_TO_REPLACE);
 	invalidate_ce_path(ce);
@@ -700,16 +739,15 @@ int threeway_merge(struct cache_entry **stages,
 	/* #14, #14ALT, #2ALT */
 	if (remote && !df_conflict_head && head_match && !remote_match) {
 		if (index && !same(index, remote) && !same(index, head))
-			reject_merge(index);
+			return o->gently ? -1 : reject_merge(index);
 		return merged_entry(remote, index, o);
 	}
 	/*
 	 * If we have an entry in the index cache, then we want to
 	 * make sure that it matches head.
 	 */
-	if (index && !same(index, head)) {
-		reject_merge(index);
-	}
+	if (index && !same(index, head))
+		return o->gently ? -1 : reject_merge(index);
 
 	if (head) {
 		/* #5ALT, #15 */
@@ -759,8 +797,10 @@ int threeway_merge(struct cache_entry **stages,
 			remove_entry(remove);
 			if (index)
 				return deleted_entry(index, index, o);
-			else if (ce && !head_deleted)
-				verify_absent(ce, "removed", o);
+			else if (ce && !head_deleted) {
+				if (verify_absent(ce, "removed", o))
+					return -1;
+			}
 			return 0;
 		}
 		/*
@@ -776,7 +816,8 @@ int threeway_merge(struct cache_entry **stages,
 	 * conflict resolution files.
 	 */
 	if (index) {
-		verify_uptodate(index, o);
+		if (verify_uptodate(index, o))
+			return -1;
 	}
 
 	remove_entry(remove);
@@ -856,11 +897,11 @@ int twoway_merge(struct cache_entry **src,
 			/* all other failures */
 			remove_entry(remove);
 			if (oldtree)
-				reject_merge(oldtree);
+				return o->gently ? -1 : reject_merge(oldtree);
 			if (current)
-				reject_merge(current);
+				return o->gently ? -1 : reject_merge(current);
 			if (newtree)
-				reject_merge(newtree);
+				return o->gently ? -1 : reject_merge(newtree);
 			return -1;
 		}
 	}
@@ -887,7 +928,8 @@ int bind_merge(struct cache_entry **src,
 		return error("Cannot do a bind merge of %d trees\n",
 			     o->merge_size);
 	if (a && old)
-		die("Entry '%s' overlaps.  Cannot bind.", a->name);
+		return o->gently ? -1 :
+			error("Entry '%s' overlaps.  Cannot bind.", a->name);
 	if (!a)
 		return keep_entry(old, o);
 	else
diff --git a/unpack-trees.h b/unpack-trees.h
index 197a0044a..a2df544d0 100644
--- a/unpack-trees.h
+++ b/unpack-trees.h
@@ -16,6 +16,8 @@ struct unpack_trees_options {
 	int trivial_merges_only;
 	int verbose_update;
 	int aggressive;
+	int skip_unmerged;
+	int gently;
 	const char *prefix;
 	int pos;
 	struct dir_struct *dir;