pack-objects: improve returned information from write_one()

pack-objects: improve returned information from write_one()

This function returns 0 when the current object couldn't be written
due to the pack size limit, otherwise the current offset in the pack.
There is a problem with this approach however, since current object
could be a delta and its delta base might just have been written in
the same write_one() call, but those successfully written objects are
not accounted in the offset variable tracked by the caller. Currently
this is not an issue but a subsequent patch will need this.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>