midx: add test that 'expire' respects .keep files

midx: add test that 'expire' respects .keep files

The 'git multi-pack-index expire' subcommand may delete packs that
are not needed from the perspective of the multi-pack-index. If
a pack has a .keep file, then we should not delete that pack. Add
a test that ensures we preserve a pack that would otherwise be
expired. First, create a new pack that contains every object in
the repo, then add it to the multi-pack-index. Then create a .keep
file for a pack starting with "a-pack" that was added in the
previous test. Finally, expire and verify that the pack remains
and the other packs were expired.

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