make git-pack-objects able to create deltas with offset to base
This is enabled with --delta-base-offset only, and doesn't work with
pack data reuse yet.
The idea is to allow for the fetch protocol to use an extension flag
to notify the remote end that --delta-base-offset can be used with
git-pack-objects. Eventually git-repack will always provide this flag.
With this, all delta base objects are now pushed before deltas that depend
on them. This is a requirements for OBJ_OFS_DELTA. This is not a
requirement for OBJ_REF_DELTA but always doing so makes the code simpler.
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>