worktree: allow "-" short-hand for @{-1} in add command

Since `git worktree add` uses `git checkout` when `[<branch>]` is used,
and `git checkout -` is already supported, it makes sense to allow the
same shortcut in `git worktree add`.

Signed-off-by: Jordan DE GEA <jordan.de-gea@grenoble-inp.org>
Signed-off-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-worktree.txt

@@ -48,7 +48,8 @@ add <path> [<branch>]::
 
 Create `<path>` and checkout `<branch>` into it. The new working directory
 is linked to the current repository, sharing everything except working
-directory specific files such as HEAD, index, etc.
+directory specific files such as HEAD, index, etc. `-` may also be
+specified as `<branch>`; it is synonymous with `@{-1}`.
 +
 If `<branch>` is omitted and neither `-b` nor `-B` nor `--detached` used,
 then, as a convenience, a new branch based at HEAD is created automatically,

builtin/worktree.c

@@ -340,6 +340,9 @@ static int add(int ac, const char **av, const char *prefix)
 	path = prefix ? prefix_filename(prefix, strlen(prefix), av[0]) : av[0];
 	branch = ac < 2 ? "HEAD" : av[1];
 
+	if (!strcmp(branch, "-"))
+		branch = "@{-1}";
+
 	opts.force_new_branch = !!new_branch_force;
 	if (opts.force_new_branch) {
 		struct strbuf symref = STRBUF_INIT;

t/t2025-worktree-add.sh

@@ -20,6 +20,22 @@ test_expect_success '"add" an existing empty worktree' '
 	git worktree add --detach existing_empty master
 '
 
+test_expect_success '"add" using shorthand - fails when no previous branch' '
+	test_must_fail git worktree add existing_short -
+'
+
+test_expect_success '"add" using - shorthand' '
+	git checkout -b newbranch &&
+	echo hello >myworld &&
+	git add myworld &&
+	git commit -m myworld &&
+	git checkout master &&
+	git worktree add short-hand - &&
+	echo refs/heads/newbranch >expect &&
+	git -C short-hand rev-parse --symbolic-full-name HEAD >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success '"add" refuses to checkout locked branch' '
 	test_must_fail git worktree add zere master &&
 	! test -d zere &&