repack: track the ODB source via existing_packs

repack: track the ODB source via existing_packs

Store the ODB source in the `existing_packs` struct and use that in
place of the raw `repo->objects->sources` access within `cmd_repack()`.

The source used is still assigned from the first source in the list, so
there are no functional changes in this commit. The changes instead
serve two purposes (one immediate, one not):

 - The incremental MIDX-based repacking machinery will need to know what
   source is being used to read the existing MIDX/chain (should one
   exist).

 - In the future, if "git repack" is taught how to operate on other
   object sources, this field will serve as the authoritative value for
   that source.

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