midx: test a few commands that use get_all_packs

midx: test a few commands that use get_all_packs

The new get_all_packs() method exposed the packfiles coverede by
a multi-pack-index. Before, the 'git cat-file --batch' and
'git count-objects' commands would skip objects in an environment
with a multi-pack-index.

Further, a reachability bitmap would be ignored if its pack-file
was covered by a multi-pack-index.

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