pack-objects: consider packs in multi-pack-index

pack-objects: consider packs in multi-pack-index

When running 'git pack-objects --local', we want to avoid packing
objects that are in an alternate. Currently, we check for these
objects using the packed_git_mru list, which excludes the pack-files
covered by a multi-pack-index.

Add a new iteration over the multi-pack-indexes to find these
copies and mark them as unwanted.

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