worktree: add: auto-vivify new branch when <branch> is omitted

As a convenience, when <branch> is omitted from "git worktree <path>
<branch>" and neither -b nor -B is used, automatically create a new
branch named after <path>, as if "-b $(basename <path>)" was specified.
Thus, "git worktree add ../hotfix" creates a new branch named "hotfix"
and associates it with new worktree "../hotfix".
Signed-off-by: NEric Sunshine <sunshine@sunshineco.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-worktree.txt

@@ -9,7 +9,7 @@ git-worktree - Manage multiple worktrees
 SYNOPSIS
 --------
 [verse]
-'git worktree add' [-f] [--detach] [-b <new-branch>] <path> <branch>
+'git worktree add' [-f] [--detach] [-b <new-branch>] <path> [<branch>]
 'git worktree prune' [-n] [-v] [--expire <expire>]
 
 DESCRIPTION
@@ -45,11 +45,15 @@ pruning should be suppressed. See section "DETAILS" for more information.
 
 COMMANDS
 --------
-add <path> <branch>::
+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.
++
+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.
 
 prune::
 

builtin/worktree.c

@@ -291,12 +291,16 @@ static int add(int ac, const char **av, const char *prefix)
 		die(_("-b and -B are mutually exclusive"));
 	if (ac < 1 || ac > 2)
 		usage_with_options(worktree_usage, options);
-	if (ac < 2 && !new_branch && !new_branch_force)
-		usage_with_options(worktree_usage, options);
 
 	path = prefix ? prefix_filename(prefix, strlen(prefix), av[0]) : av[0];
 	branch = ac < 2 ? "HEAD" : av[1];
 
+	if (ac < 2 && !new_branch && !new_branch_force) {
+		int n;
+		const char *s = worktree_basename(path, &n);
+		new_branch = xstrndup(s, n);
+	}
+
 	argv_array_push(&cmd, "checkout");
 	if (force)
 		argv_array_push(&cmd, "--ignore-other-worktrees");

t/t2025-worktree-add.sh

@@ -145,4 +145,18 @@ test_expect_success '"add -b" with <branch> omitted' '
 	test_cmp_rev HEAD burble
 '
 
+test_expect_success '"add" with <branch> omitted' '
+	git worktree add wiffle/bat &&
+	test_cmp_rev HEAD bat
+'
+
+test_expect_success '"add" auto-vivify does not clobber existing branch' '
+	test_commit c1 &&
+	test_commit c2 &&
+	git branch precious HEAD~1 &&
+	test_must_fail git worktree add precious &&
+	test_cmp_rev HEAD~1 precious &&
+	test_path_is_missing precious
+'
+
 test_done