delete_ref(): delete packed ref

delete_ref(): delete packed ref

This implements deletion of a packed ref.  Since it is a very
rare event to delete a ref compared to looking up, creating and
updating, this opts to remove the ref from the packed-ref file
instead of doing any of the filesystem based "negative ref" trick
to optimize the deletion path.

Signed-off-by: Junio C Hamano <junkio@cox.net>