This adds bare-bone completion support for git-worktree. More advanced
completion (e.g. ref completion in git-worktree-add) can be added later.
--force completion in "worktree add" is left out because that option
should be handled with care.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Reviewed-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>