Make git-cherry handle root trees

This patch on top of 'next' makes built-in git-cherry handle root
commits.

It moves the static function log-tree.c::diff_root_tree() to
tree-diff.c and makes it more similar to diff_tree_sha1() by
shuffling around arguments and factoring out the call to
log_tree_diff_flush().  Consequently the name is changed to
diff_root_tree_sha1().  It is a version of diff_tree_sha1() that
compares the empty tree (= root tree) against a single 'real' tree.

This function is then used in get_patch_id() to compute patch IDs
for initial commits instead of SEGFAULTing, as the current code
does if confronted with parentless commits.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 18 insertions, 0 deletions

diff --git a/tree-diff.c b/tree-diff.c
index 7e2f4f088..37d235e06 100644
--- a/tree-diff.c
+++ b/tree-diff.c
@@ -215,6 +215,24 @@ int diff_tree_sha1(const unsigned char *old, const unsigned char *new, const cha
 	return retval;
 }
 
+int diff_root_tree_sha1(const unsigned char *new, const char *base, struct diff_options *opt)
+{
+	int retval;
+	void *tree;
+	struct tree_desc empty, real;
+
+	tree = read_object_with_reference(new, tree_type, &real.size, NULL);
+	if (!tree)
+		die("unable to read root tree (%s)", sha1_to_hex(new));
+	real.buf = tree;
+
+	empty.size = 0;
+	empty.buf = "";
+	retval = diff_tree(&empty, &real, base, opt);
+	free(tree);
+	return retval;
+}
+
 static int count_paths(const char **paths)
 {
 	int i = 0;