Start using multi-pack bitmaps

Until Git v2.34.0, a repository could've only had a single bitmap. As a result, when object pools are in use, then the bitmap is only created for the pool repository and never for the forks. This is really inefficient given that we'd instead want to have per-repository as well as per-pool bitmaps.

This limitation was removed in Git v2.34.0, so we should start to make use of multi-pack bitmaps. See https://github.blog/2021-11-15-highlights-from-git-2-34/#multi-pack-reachability-bitmaps.