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

@@ -68,6 +68,7 @@ OPTIONS
 -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>`.