worktree: add: suppress auto-vivication with --detach and no <branch>

Fix oversight where branch auto-vivication incorrectly kicks in when
--detach is specified and <branch> omitted. Instead, treat:

    git worktree add --detach <path>

as shorthand for:

    git worktree add --detach <path> HEAD

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-worktree.txt

@@ -51,9 +51,9 @@ 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.
 +
-If `<branch>` is omitted and neither `-b` nor `-B` is used, then, as a
-convenience, a new branch based at HEAD is created automatically, as if
-`-b $(basename <path>)` was specified.
+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,
+as if `-b $(basename <path>)` was specified.
 
 prune::
 

builtin/worktree.c

@@ -308,7 +308,7 @@ static int add(int ac, const char **av, const char *prefix)
 	if (opts.force_new_branch)
 		opts.new_branch = new_branch_force;
 
-	if (ac < 2 && !opts.new_branch) {
+	if (ac < 2 && !opts.new_branch && !opts.detach) {
 		int n;
 		const char *s = worktree_basename(path, &n);
 		opts.new_branch = xstrndup(s, n);

t/t2025-worktree-add.sh

@@ -153,6 +153,14 @@ test_expect_success '"add -b" with <branch> omitted' '
 	test_cmp_rev HEAD burble
 '
 
+test_expect_success '"add --detach" with <branch> omitted' '
+	git worktree add --detach fishhook &&
+	git rev-parse HEAD >expected &&
+	git -C fishhook rev-parse HEAD >actual &&
+	test_cmp expected actual &&
+	test_must_fail git -C fishhook symbolic-ref HEAD
+'
+
 test_expect_success '"add" with <branch> omitted' '
 	git worktree add wiffle/bat &&
 	test_cmp_rev HEAD bat
@@ -167,6 +175,12 @@ test_expect_success '"add" auto-vivify does not clobber existing branch' '
 	test_path_is_missing precious
 '
 
+test_expect_success '"add" no auto-vivify with --detach and <branch> omitted' '
+	git worktree add --detach mish/mash &&
+	test_must_fail git rev-parse mash -- &&
+	test_must_fail git -C mish/mash symbolic-ref HEAD
+'
+
 test_expect_success '"add" -b/-B mutually exclusive' '
 	test_must_fail git worktree add -b poodle -B poodle bamboo master
 '