pack-bitmap: consolidate `find_object_pos()` success path

pack-bitmap: consolidate `find_object_pos()` success path

Both sides of `find_object_pos()` report success in the same way by
setting the optional `found` out-parameter and return the resolved
bitmap position.

Prepare for adding more bookkeeping around object-position lookups by
storing the result in a local `pos` variable and sharing the success
return path between the packlist and MIDX cases.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>