[PATCH] worktree add: introduce basic DWYM for --orphan

[Jacob Abel <jacobabel@nullpo.dev>]

Introduces a DWYM shorthand of --orphan for when the worktree directory
and the to-be-created branch share the same name.

Current Behavior:
    % git worktree list
    /path/to/git/repo        a38d39a4c5 [main]
    % git worktree add --orphan new_branch ../new_branch/
    Preparing worktree (new branch 'new_branch')
    % git worktree add --orphan ../new_branch2/
    usage: git worktree add [<options>] <path> [<commit-ish>]
       or: git worktree list [<options>]
    [...]
    %

New Behavior:

    % git worktree list
    /path/to/git/repo        a38d39a4c5 [main]
    % git worktree add --orphan new_branch ../new_branch/
    Preparing worktree (new branch 'new_branch')
    % git worktree list
    /path/to/git/repo        a38d39a4c5 [main]
    /path/to/git/new_branch  a38d39a4c5 [new_branch]
    % git worktree add --orphan ../new_branch2/
    Preparing worktree (new branch 'new_branch2')
    % git worktree list
    /path/to/git/repo        a38d39a4c5 [main]
    /path/to/git/new_branch  a38d39a4c5 [new_branch]
    /path/to/git/new_branch2 a38d39a4c5 [new_branch2]
    %

Signed-off-by: Jacob Abel <jacobabel@nullpo.dev>
---
 Documentation/git-worktree.txt | 13 +++++++++----
 builtin/worktree.c             | 21 +++++++++++++++------
 t/t2400-worktree-add.sh        | 10 ++++++++++
 3 files changed, 34 insertions(+), 10 deletions(-)

diff --git a/Documentation/git-worktree.txt b/Documentation/git-worktree.txt
index d78460c29c..a56ddb0185 100644
--- a/Documentation/git-worktree.txt
+++ b/Documentation/git-worktree.txt
@@ -12,7 +12,7 @@ SYNOPSIS
 'git worktree add' [-f] [--detach] [--checkout] [--lock [--reason <string>]]
 		   [(-b | -B) <new-branch>] <path> [<commit-ish>]
 'git worktree add' [-f] [--lock [--reason <string>]]
-		   --orphan <new-branch> <path>
+		   --orphan [<new-branch>] <path>
 'git worktree list' [-v | --porcelain [-z]]
 'git worktree lock' [--reason <string>] <worktree>
 'git worktree move' <worktree> <new-path>
@@ -99,13 +99,16 @@ in the new worktree, if it's not checked out anywhere else, otherwise the
 command will refuse to create the worktree (unless `--force` is used).
 +
 ------------
-$ git worktree add --orphan <branch> <path>
+$ git worktree add --orphan [<branch>] <path>
 ------------
 +
 Create a worktree containing no files, with an empty index, and associated
 with a new orphan branch named `<branch>`. The first commit made on this new
 branch will have no parents and will be the root of a new history disconnected
 from any other branches.
++
+If a branch name `<branch>` is not supplied, the name is derived from the
+supplied path `<path>`.

 list::

@@ -233,9 +236,11 @@ This can also be set up as the default behaviour by using the
 	With `prune`, do not remove anything; just report what it would
 	remove.

---orphan <new-branch>::
+--orphan [<new-branch>]::
 	With `add`, make the new worktree and index empty, associating
-	the worktree with a new orphan branch named `<new-branch>`.
+	the worktree with a new orphan branch named `<new-branch>`. If
+	`<new-branch>` is not supplied, the new branch name is derived
+	from `<path>`.

 --porcelain::
 	With `list`, output in an easy-to-parse format for scripts.
diff --git a/builtin/worktree.c b/builtin/worktree.c
index d975628353..481f895075 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -19,7 +19,7 @@
 	N_("git worktree add [-f] [--detach] [--checkout] [--lock [--reason <string>]]\n" \
 	   "                 [(-b | -B) <new-branch>] <path> [<commit-ish>]"), \
 	N_("git worktree add [-f] [--lock [--reason <string>]]\n" \
-	   "                 --orphan <new-branch> <path>")
+	   "                 --orphan [<new-branch>] <path>")

 #define BUILTIN_WORKTREE_LIST_USAGE \
 	N_("git worktree list [-v | --porcelain [-z]]")
@@ -681,10 +681,13 @@ static int add(int ac, const char **av, const char *prefix)
 	else if (keep_locked)
 		opts.keep_locked = _("added with --lock");

-	if (ac < 1 || ac > 2)
+	if (ac < 1 && opts.orphan) {
+		path = prefix_filename(prefix, orphan_branch);
+	} else if (ac >= 1 && ac <= 2) {
+		path = prefix_filename(prefix, av[0]);
+	} else {
 		usage_with_options(git_worktree_add_usage, options);
-
-	path = prefix_filename(prefix, av[0]);
+	}
 	branch = ac < 2 ? "HEAD" : av[1];

 	if (!strcmp(branch, "-"))
@@ -702,14 +705,20 @@ static int add(int ac, const char **av, const char *prefix)
 		strbuf_release(&symref);
 	}

-	if (opts.orphan) {
-		new_branch = orphan_branch;
+	if (ac < 1 && opts.orphan) {
+		const char *s = dwim_branch(path, &orphan_branch);
+		if (s)
+			orphan_branch = s;
 	} else if (ac < 2 && !new_branch && !opts.detach) {
 		const char *s = dwim_branch(path, &new_branch);
 		if (s)
 			branch = s;
 	}

+	if (opts.orphan) {
+		new_branch = orphan_branch;
+	}
+
 	if (ac == 2 && !new_branch && !opts.detach) {
 		struct object_id oid;
 		struct commit *commit;
diff --git a/t/t2400-worktree-add.sh b/t/t2400-worktree-add.sh
index 1bf8d619e2..c3de277738 100755
--- a/t/t2400-worktree-add.sh
+++ b/t/t2400-worktree-add.sh
@@ -354,6 +354,16 @@ test_expect_success '"add --orphan" fails if the branch already exists' '
 	test_path_is_missing orphandir2
 '

+test_expect_success '"add --orphan" with basic DWYM' '
+	test_when_finished "rm -rf empty_repo" &&
+	echo refs/heads/worktreedir >expected &&
+	GIT_DIR="empty_repo" git init --bare &&
+	# Use non-trivial path to verify it DWYMs properly.
+	git -C empty_repo worktree add --orphan ../empty_repo/worktreedir &&
+	git -C empty_repo/worktreedir symbolic-ref HEAD >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success '"add --orphan" with empty repository' '
 	test_when_finished "rm -rf empty_repo" &&
 	echo refs/heads/newbranch >expected &&
--
2.38.2