LoadBlobsFromPack is now part of the repository struct. This ensures
that users of that method don't have to deal will internals of the
repository implementation.
The filerestorer tests now also contain far fewer pack file
implementation details.
The test now uses the fact that the sort is stable. It's not guaranteed
to be, but the test is cleaner and more exhaustive. sortCachedPacksFirst
no longer needs a return value.
