branch --set-upstream: regression fix

The "git branch" command, while not in listing mode, calls create_branch()
even when the target branch already exists, and it does so even when it is
not interested in updating the value of the branch (i.e. the name of the
commit object that sits at the tip of the existing branch). This happens
when the command is run with "--set-upstream" option.

The earlier safety-measure to prevent "git branch -f $branch $commit" from
updating the currently checked out branch did not take it into account,
and we no longer can update the tracking information of the current branch.

Minimally fix this regression by telling the validation code if it is
called to really update the value of a potentially existing branch, or if
the caller merely is interested in updating auxiliary aspects of a branch.

Reported-and-Tested-by: Jay Soffian
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 12 insertions, 9 deletions

diff --git a/branch.c b/branch.c
index 1fe3078a6..4338a90b3 100644
--- a/branch.c
+++ b/branch.c
@@ -135,23 +135,25 @@ static int setup_tracking(const char *new_ref, const char *orig_ref,
 	return 0;
 }
 
-int validate_new_branchname(const char *name, struct strbuf *ref, int force)
+int validate_new_branchname(const char *name, struct strbuf *ref,
+			    int force, int attr_only)
 {
-	const char *head;
-	unsigned char sha1[20];
-
 	if (strbuf_check_branch_ref(ref, name))
 		die("'%s' is not a valid branch name.", name);
 
 	if (!ref_exists(ref->buf))
 		return 0;
-	else if (!force)
+	else if (!force && !attr_only)
 		die("A branch named '%s' already exists.", ref->buf + strlen("refs/heads/"));
 
-	head = resolve_ref("HEAD", sha1, 0, NULL);
-	if (!is_bare_repository() && head && !strcmp(head, ref->buf))
-		die("Cannot force update the current branch.");
+	if (!attr_only) {
+		const char *head;
+		unsigned char sha1[20];
 
+		head = resolve_ref("HEAD", sha1, 0, NULL);
+		if (!is_bare_repository() && head && !strcmp(head, ref->buf))
+			die("Cannot force update the current branch.");
+	}
 	return 1;
 }
 
@@ -171,7 +173,8 @@ void create_branch(const char *head,
 	if (track == BRANCH_TRACK_EXPLICIT || track == BRANCH_TRACK_OVERRIDE)
 		explicit_tracking = 1;
 
-	if (validate_new_branchname(name, &ref, force || track == BRANCH_TRACK_OVERRIDE)) {
+	if (validate_new_branchname(name, &ref, force,
+				    track == BRANCH_TRACK_OVERRIDE)) {
 		if (!force)
 			dont_change_ref = 1;
 		else