midx: during verify group objects by packfile to speed verification

midx: during verify group objects by packfile to speed verification

Teach `multi-pack-index verify` to sort the set of object by
packfile so that only one packfile needs to be open at a time.

This is a performance improvement.  Previously, objects were
verified in OID order.  This essentially requires all packfiles
to be open at the same time.  If the number of packfiles exceeds
the open file limit, packfiles would be LRU-closed and re-opened
many times.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>