Add branch --set-upstream

Add --set-upstream option to branch that works like --track, except that
when branch exists already, its upstream info is changed without changing
the ref value.

Based-on-patch-from: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-branch.txt

@@ -11,7 +11,7 @@ SYNOPSIS
 'git branch' [--color | --no-color] [-r | -a]
 	[-v [--abbrev=<length> | --no-abbrev]]
 	[(--merged | --no-merged | --contains) [<commit>]]
-'git branch' [--track | --no-track] [-l] [-f] <branchname> [<start-point>]
+'git branch' [--set-upstream | --track | --no-track] [-l] [-f] <branchname> [<start-point>]
 'git branch' (-m | -M) [<oldbranch>] <newbranch>
 'git branch' (-d | -D) [-r] <branchname>...
 
@@ -129,6 +129,12 @@ start-point is either a local or remote branch.
 	Do not set up "upstream" configuration, even if the
 	branch.autosetupmerge configuration variable is true.
 
+--set-upstream::
+	If specified branch does not exist yet or if '--force' has been
+	given, acts exactly like '--track'. Otherwise sets up configuration
+	like '--track' would when creating the branch, except that where
+	branch points to is not changed.
+
 --contains <commit>::
 	Only list branches which contain the specified commit.
 

branch.c

@@ -108,6 +108,7 @@ static int setup_tracking(const char *new_ref, const char *orig_ref,
 		switch (track) {
 		case BRANCH_TRACK_ALWAYS:
 		case BRANCH_TRACK_EXPLICIT:
+		case BRANCH_TRACK_OVERRIDE:
 			break;
 		default:
 			return 1;
@@ -128,18 +129,25 @@ void create_branch(const char *head,
 		   const char *name, const char *start_name,
 		   int force, int reflog, enum branch_track track)
 {
-	struct ref_lock *lock;
+	struct ref_lock *lock = NULL;
 	struct commit *commit;
 	unsigned char sha1[20];
 	char *real_ref, msg[PATH_MAX + 20];
 	struct strbuf ref = STRBUF_INIT;
 	int forcing = 0;
+	int dont_change_ref = 0;
+	int explicit_tracking = 0;
+
+	if (track == BRANCH_TRACK_EXPLICIT || track == BRANCH_TRACK_OVERRIDE)
+		explicit_tracking = 1;
 
 	if (strbuf_check_branch_ref(&ref, name))
 		die("'%s' is not a valid branch name.", name);
 
 	if (resolve_ref(ref.buf, sha1, 1, NULL)) {
-		if (!force)
+		if (!force && track == BRANCH_TRACK_OVERRIDE)
+			dont_change_ref = 1;
+		else 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.");
@@ -153,12 +161,12 @@ void create_branch(const char *head,
 	switch (dwim_ref(start_name, strlen(start_name), sha1, &real_ref)) {
 	case 0:
 		/* Not branching from any existing branch */
-		if (track == BRANCH_TRACK_EXPLICIT)
+		if (explicit_tracking)
 			die("Cannot setup tracking information; starting point is not a branch.");
 		break;
 	case 1:
 		/* Unique completion -- good, only if it is a real ref */
-		if (track == BRANCH_TRACK_EXPLICIT && !strcmp(real_ref, "HEAD"))
+		if (explicit_tracking && !strcmp(real_ref, "HEAD"))
 			die("Cannot setup tracking information; starting point is not a branch.");
 		break;
 	default:
@@ -170,9 +178,11 @@ void create_branch(const char *head,
 		die("Not a valid branch point: '%s'.", start_name);
 	hashcpy(sha1, commit->object.sha1);
 
-	lock = lock_any_ref_for_update(ref.buf, NULL, 0);
-	if (!lock)
-		die_errno("Failed to lock ref for update");
+	if (!dont_change_ref) {
+		lock = lock_any_ref_for_update(ref.buf, NULL, 0);
+		if (!lock)
+			die_errno("Failed to lock ref for update");
+	}
 
 	if (reflog)
 		log_all_ref_updates = 1;
@@ -180,15 +190,16 @@ void create_branch(const char *head,
 	if (forcing)
 		snprintf(msg, sizeof msg, "branch: Reset from %s",
 			 start_name);
-	else
+	else if (!dont_change_ref)
 		snprintf(msg, sizeof msg, "branch: Created from %s",
 			 start_name);
 
 	if (real_ref && track)
 		setup_tracking(name, real_ref, track);
 
-	if (write_ref_sha1(lock, sha1, msg) < 0)
-		die_errno("Failed to write ref");
+	if (!dont_change_ref)
+		if (write_ref_sha1(lock, sha1, msg) < 0)
+			die_errno("Failed to write ref");
 
 	strbuf_release(&ref);
 	free(real_ref);

builtin-branch.c

@@ -564,6 +564,8 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 		OPT__VERBOSE(&verbose),
 		OPT_SET_INT('t', "track",  &track, "set up tracking mode (see git-pull(1))",
 			BRANCH_TRACK_EXPLICIT),
+		OPT_SET_INT( 0, "set-upstream",  &track, "change upstream info",
+			BRANCH_TRACK_OVERRIDE),
 		OPT_BOOLEAN( 0 , "color",  &branch_use_color, "use colored output"),
 		OPT_SET_INT('r', NULL,     &kinds, "act on remote-tracking branches",
 			REF_REMOTE_BRANCH),

cache.h

@@ -553,6 +553,7 @@ enum branch_track {
 	BRANCH_TRACK_REMOTE,
 	BRANCH_TRACK_ALWAYS,
 	BRANCH_TRACK_EXPLICIT,
+	BRANCH_TRACK_OVERRIDE,
 };
 
 enum rebase_setup_type {

t/t6040-tracking-info.sh

@@ -89,4 +89,25 @@ test_expect_success 'status when tracking annotated tags' '
 	grep "set up to track" actual &&
 	git checkout heavytrack
 '
+
+test_expect_success 'setup tracking with branch --set-upstream on existing branch' '
+	git branch from-master master &&
+	test_must_fail git config branch.from-master.merge > actual &&
+	git branch --set-upstream from-master master &&
+	git config branch.from-master.merge > actual &&
+	grep -q "^refs/heads/master$" actual
+'
+
+test_expect_success '--set-upstream does not change branch' '
+	git branch from-master2 master &&
+	test_must_fail git config branch.from-master2.merge > actual &&
+	git rev-list from-master2 &&
+	git update-ref refs/heads/from-master2 from-master2^ &&
+	git rev-parse from-master2 >expect2 &&
+	git branch --set-upstream from-master2 master &&
+	git config branch.from-master.merge > actual &&
+	git rev-parse from-master2 >actual2 &&
+	grep -q "^refs/heads/master$" actual &&
+	cmp expect2 actual2
+'
 test_done