branch: error message checking out a branch in use

Let's update the error message we show when the user tries to check out
a branch which is being used in another worktree, following the
guideline reasoned in 4970bedef2 (branch: update the message to refuse
touching a branch in-use, 2023-07-21).

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

branch.c

@@ -838,7 +838,7 @@ void die_if_checked_out(const char *branch, int ignore_current_worktree)
 
 		if (is_shared_symref(worktrees[i], "HEAD", branch)) {
 			skip_prefix(branch, "refs/heads/", &branch);
-			die(_("'%s' is already checked out at '%s'"),
+			die(_("'%s' is already used by worktree at '%s'"),
 				branch, worktrees[i]->path);
 		}
 	}

t/t2400-worktree-add.sh

@@ -121,7 +121,8 @@ test_expect_success '"add" worktree creating new branch' '
 test_expect_success 'die the same branch is already checked out' '
 	(
 		cd here &&
-		test_must_fail git checkout newmain
+		test_must_fail git checkout newmain 2>actual &&
+		grep "already used by worktree at" actual
 	)
 '
 

t/t3400-rebase.sh

@@ -421,7 +421,7 @@ test_expect_success 'refuse to switch to branch checked out elsewhere' '
 	git checkout main &&
 	git worktree add wt &&
 	test_must_fail git -C wt rebase main main 2>err &&
-	test_i18ngrep "already checked out" err
+	test_i18ngrep "already used by worktree at" err
 '
 
 test_expect_success MINGW,SYMLINKS_WINDOWS 'rebase when .git/logs is a symlink' '