worktree: add: make -b/-B default to HEAD when <branch> is omitted

As a convenience, like "git branch" and "git checkout -b", make
"git worktree add -b <newbranch> <path> <branch>" default to HEAD when
<branch> is omitted.

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

Documentation/git-worktree.txt

diff --git a/Documentation/git-worktree.txt b/Documentation/git-worktree.txt
index f44cd78..377ae0f 100644
--- a/Documentation/git-worktree.txt
+++ b/Documentation/git-worktree.txt
@@ -68,6 +68,7 @@
 -B <new-branch>::
 	With `add`, create a new branch named `<new-branch>` starting at
 	`<branch>`, and check out `<new-branch>` into the new worktree.
+	If `<branch>` is omitted, it defaults to HEAD.
 	By default, `-b` refuses to create a new branch if it already
 	exists. `-B` overrides this safeguard, resetting `<new-branch>` to
 	`<branch>`.

builtin/worktree.c

diff --git a/builtin/worktree.c b/builtin/worktree.c
index 25fe25b..323d444 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -289,11 +289,13 @@
 	ac = parse_options(ac, av, prefix, options, worktree_usage, 0);
 	if (new_branch && new_branch_force)
 		die(_("-b and -B are mutually exclusive"));
-	if (ac != 2)
+	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 = av[1];
+	branch = ac < 2 ? "HEAD" : av[1];
 
 	argv_array_push(&cmd, "checkout");
 	if (force)

t/t2025-worktree-add.sh

diff --git a/t/t2025-worktree-add.sh b/t/t2025-worktree-add.sh
index 192c936..8964dec 100755
--- a/t/t2025-worktree-add.sh
+++ b/t/t2025-worktree-add.sh
@@ -140,4 +140,9 @@
 	test_cmp expected actual
 '
 
+test_expect_success '"add -b" with <branch> omitted' '
+	git worktree add -b burble flornk &&
+	test_cmp_rev HEAD burble
+'
+
 test_done