t7700: test that 'repack -a' packs alternate packed objects

t7700: test that 'repack -a' packs alternate packed objects

Previously, when 'repack -a' was called and there were no packs in the local
repository without a .keep file, the repack would fall back to calling
pack-objects with '--unpacked --incremental'. This resulted in the created
pack file, if any, to be missing the packed objects in the alternate object
store. Test that this specific case has been fixed.

Signed-off-by: Brandon Casey <redacted>
Signed-off-by: Junio C Hamano <redacted>