branch: rename orphan branches in any worktree

In cfaff3a (branch -m: allow renaming a yet-unborn branch, 2020-12-13)
we added support for renaming an orphan branch when that branch is
checked out in the current worktree.

Let's also allow renaming an orphan branch checked out in a worktree
different than the current one.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -539,6 +539,7 @@ static int replace_each_worktree_head_symref(struct worktree **worktrees,
 }
 
 #define IS_HEAD 1
+#define IS_ORPHAN 2
 
 static void copy_or_rename_branch(const char *oldname, const char *newname, int copy, int force)
 {
@@ -565,6 +566,8 @@ static void copy_or_rename_branch(const char *oldname, const char *newname, int
 
 		if (wt->head_ref && !strcmp(oldref.buf, wt->head_ref)) {
 			oldref_usage |= IS_HEAD;
+			if (is_null_oid(&wt->head_oid))
+				oldref_usage |= IS_ORPHAN;
 			break;
 		}
 	}
@@ -599,8 +602,7 @@ static void copy_or_rename_branch(const char *oldname, const char *newname, int
 		strbuf_addf(&logmsg, "Branch: renamed %s to %s",
 			    oldref.buf, newref.buf);
 
-	if (!copy &&
-	    (!head || strcmp(oldname, head) || !is_null_oid(&head_oid)) &&
+	if (!copy && !(oldref_usage & IS_ORPHAN) &&
 	    rename_ref(oldref.buf, newref.buf, logmsg.buf))
 		die(_("Branch rename failed"));
 	if (copy && copy_existing_ref(oldref.buf, newref.buf, logmsg.buf))

t/t3200-branch.sh

@@ -294,6 +294,20 @@ test_expect_success 'git branch -M and -C fail on detached HEAD' '
 	test_cmp expect err
 '
 
+test_expect_success 'git branch -m should work with orphan branches' '
+	test_when_finished git checkout - &&
+	test_when_finished git worktree remove -f wt &&
+	git worktree add wt --detach &&
+	# rename orphan in another worktreee
+	git -C wt checkout --orphan orphan-foo-wt &&
+	git branch -m orphan-foo-wt orphan-bar-wt &&
+	test orphan-bar-wt=$(git -C orphan-worktree branch --show-current) &&
+	# rename orphan in the current worktree
+	git checkout --orphan orphan-foo &&
+	git branch -m orphan-foo orphan-bar &&
+	test orphan-bar=$(git branch --show-current)
+'
+
 test_expect_success 'git branch -d on orphan HEAD (merged)' '
 	test_when_finished git checkout main &&
 	git checkout --orphan orphan &&