git/housekeeping: Don't loosen objects when eagerly packing object pools

When eagerly repacking object pools we use a strategy that repacks all reachable objects into one pack, which is what we want. But we also explode unreachable objects into loose objects, which does not make any sense at all as we wouldn't ever delete these unreachable objects anyway.

This really is a theoretical problem, only: there exist no callers that invoke the eager optimization strategy on object pools. But let's fix it regardless of that and instead use a strategy where unreachable objects will instead be appended to the packfile.