midx: fix bug that skips midx with alternates

The logic for constructing the linked list of multi-pack-indexes
in the object store is incorrect. If the local object store has
a multi-pack-index, but an alternate does not, then the list is
dropped.

Add tests that would have revealed this bug.

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