packfile: add all_packs list

packfile: add all_packs list

If a repo contains a multi-pack-index, then the packed_git list
does not contain the packfiles that are covered by the multi-pack-index.
This is important for doing object lookups, abbreviations, and
approximating object count. However, there are many operations that
really want to iterate over all packfiles.

Create a new 'all_packs' linked list that contains this list, starting
with the packfiles in the multi-pack-index and then continuing along
the packed_git linked list.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>