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>