Two subcommands are added for completion: merge and get-ref. get-ref
is more like plumbing. But since it does not share the prefix with any
other subcommands, it won't slow anybody down.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>