This is the missing part to git-pack-objects allowing it to reuse delta
data to/from any of the two delta types.  It can reuse delta from any
type, and it outputs base offsets when --allow-delta-base-offset is
provided and the base is also included in the pack.  Otherwise it
outputs base sha1 references just like it always did.
Signed-off-by: NNicolas Pitre <nico@cam.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>