let the GIT native protocol use offsets to delta base when possible

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>