packfile: always populate pack-specific info when reading object info

packfile: always populate pack-specific info when reading object info

When reading object information via `packed_object_info()` we may not
populate the object info's packfile-specific fields. This leads to
inconsistent object info depending on whether the info was populated via
`packfile_store_read_object_info()` or `packed_object_info()`.

Fix this inconsistency so that we can always assume the pack info to be
populated when reading object info from a pack.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>