allow for undeltified objects not to be reused

allow for undeltified objects not to be reused

Currently non deltified object data is always reused when possible.
This means that any change to core.compression has no effect on those
objects as they don't get recompressed when repacking them.

Let's add a --no-reuse-object flag to git-repack in order to force
recompression of all objects when desired.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>