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>