packfile: only prepare owning store in `packfile_store_get_packs()`

packfile: only prepare owning store in `packfile_store_get_packs()`

When calling `packfile_store_get_packs()` we prepare not only the
provided packfile store, but also all those of all other sources part of
the same object database. This was required when the store was still
sitting on the object database level. But now that it sits on the source
level it's not anymore.

Adapt the code so that we only prepare the MIDX of the provided store.
All callers only work in the context of a single store or call the
function in a loop over all sources, so this change shouldn't have any
practical effects.

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