multi-pack-index: test expire while adding packs

multi-pack-index: test expire while adding packs

During development of the multi-pack-index expire subcommand, a
version went out that improperly computed the pack order if a new
pack was introduced while other packs were being removed. Part of
the subtlety of the bug involved the new pack being placed before
other packs that already existed in the multi-pack-index.

Add a test to t5319-multi-pack-index.sh that catches this issue.
The test adds new packs that cause another pack to be expired, and
creates new packs that are lexicographically sorted before and
after the existing packs.

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