teach update-ref a "--rename" option

You can rename a branch with "branch -m", but there is no
plumbing mechanism to rename an arbitrary ref. You can
simulate it with:

  git update-ref newname oldname &&
  git update-ref -d oldname

but that drops the reflog from oldname. This patch instead
allows:

  git update-ref --rename oldname newname

Unlike "git branch -m", it does not munge the branch config
(since this plumbing command is purely about refs), nor does
it allow a missing "oldname" to mean an implicit HEAD
(because it is plumbing, and we are better to make the
caller be explicit than risk a surprise).

Signed-off-by: Jeff King <peff@peff.net>

Author: peff

builtin/update-ref.c

@@ -15,6 +15,7 @@
 static const char * const git_update_ref_usage[] = {
 	N_("git update-ref [<options>] -d <refname> [<old-oid>]"),
 	N_("git update-ref [<options>]    <refname> <new-oid> [<old-oid>]"),
+	N_("git update-ref [<options>] --rename <refname> <refname>"),
 	N_("git update-ref [<options>] --stdin [-z] [--batch-updates]"),
 	NULL
 };
@@ -25,6 +26,19 @@ static unsigned int default_flags;
 static unsigned create_reflog_flag;
 static const char *msg;
 
+static int do_rename_ref(const char *from, const char *to)
+{
+	struct strbuf msg = STRBUF_INIT;
+	int ret;
+
+	strbuf_addf(&msg, "update-ref: renamed %s to %s", from, to);
+	ret = refs_rename_ref(get_main_ref_store(the_repository),
+			      from, to, msg.buf);
+	strbuf_release(&msg);
+
+	return !!ret;
+}
+
 /*
  * Parse one whitespace- or NUL-terminated, possibly C-quoted argument
  * and append the result to arg.  Return a pointer to the terminator.
@@ -759,10 +773,12 @@ int cmd_update_ref(int argc,
 	int delete = 0, no_deref = 0, read_stdin = 0, end_null = 0;
 	int create_reflog = 0;
 	unsigned int flags = 0;
+	int rename = 0;
 
 	struct option options[] = {
 		OPT_STRING( 'm', NULL, &msg, N_("reason"), N_("reason of the update")),
 		OPT_BOOL('d', NULL, &delete, N_("delete the reference")),
+		OPT_BOOL( 0 , "rename", &rename, N_("rename the reference")),
 		OPT_BOOL( 0 , "no-deref", &no_deref,
 					N_("update <refname> not the one it points to")),
 		OPT_BOOL('z', NULL, &end_null, N_("stdin has NUL-terminated arguments")),
@@ -787,7 +803,7 @@ int cmd_update_ref(int argc,
 	}
 
 	if (read_stdin) {
-		if (delete || argc > 0)
+		if (delete || rename || argc > 0)
 			usage_with_options(git_update_ref_usage, options);
 		if (end_null)
 			line_termination = '\0';
@@ -800,6 +816,12 @@ int cmd_update_ref(int argc,
 	if (end_null)
 		usage_with_options(git_update_ref_usage, options);
 
+	if (rename) {
+		if (delete || argc < 2 || argc > 2)
+			usage_with_options(git_update_ref_usage, options);
+		return do_rename_ref(argv[0], argv[1]);
+	}
+
 	if (delete) {
 		if (argc < 1 || argc > 2)
 			usage_with_options(git_update_ref_usage, options);

t/t1400-update-ref.sh

@@ -12,6 +12,7 @@ Z=$ZERO_OID
 m=refs/heads/main
 outside=refs/foo
 bare=bare-repo
+d=refs/heads/dst
 
 create_test_commits ()
 {
@@ -424,8 +425,30 @@ test_expect_success 'Query "main@{2005-05-28}" (past end of history)' '
 	test_grep -F "warning: log for ref $m unexpectedly ended on $ld" e
 '
 
+test_expect_success 'rename a ref' '
+	git rev-parse --verify $m >expect &&
+
+	# set the date to match the reflog entries we created
+	# above, which do not follow test_tick; otherwise
+	# the we write an out-of-order entry into the reflog,
+	# which confuses the reflog parser
+	GIT_COMMITTER_DATE=$ld \
+	git update-ref --rename $m $d &&
+
+	test_must_fail git rev-parse --verify $m &&
+	git rev-parse --verify $d >o &&
+	test_cmp expect o
+'
+
+test_expect_success 'renames copy reflogs' '
+	echo "$C" >expect &&
+	git rev-parse --verify "$d@{2005-05-26 23:32:00}" >o &&
+	test_cmp expect o
+'
+
 rm -f expect
 git update-ref -d $m
+git update-ref -d $d
 
 test_expect_success 'query reflog with gap' '
 	test_when_finished "git update-ref -d $m" &&