Cleanup unpack_object_header to use only offsets.

Cleanup unpack_object_header to use only offsets.

If we're always incrementing both the offset and the pointer we
aren't gaining anything by keeping both.  Instead just use the
offset since that's what we were given and what we are expected
to return.  Also using offset is likely to make it easier to remap
the pack in the future should partial mapping of very large packs
get implemented.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>