branchcache: stop storing a repository instance on the cache altogether We did not really needed it and we do not needs it anymore at all. So lets make things simpler for consistency and garbage collecting and stop storing it altogether.

branchcache: pass the target repository when copying Branchmap are usually copied to be used on a different repoview using a different filter level. Passing the repository around means the repository in `branchcache._repo` will drift from the actual branchmap filter. This is currently "fine" because the repo is only used to retrieve the `nullid` value. However, this is a fairly big trap for any extension or future code using the `_repo` attribute. The replace logic is now using a copy to ensure the right repository view is used to initialized the cached value. We add a couple of assert for make sure this inconsistency does not sneak back.

branchcache: have an explicit method to update the on disk cache Explicit is better and will give use more flexibility for future evolution of the storage.