let the GIT native protocol use offsets to delta base when possible
There is no reason not to always do this when both ends agree.
Therefore a client that can accept offsets to delta base always sends
the "ofs-delta" flag. The server will stream a pack with or without
offset to delta base depending on whether that flag is provided or not
with no additional cost.
Signed-off-by: Nicolas Pitre <nico@cam.org> Signed-off-by: Junio C Hamano <junkio@cox.net>