Support for hashing external repos

While trying to debug https://github.com/Tinder/bazel-diff/issues/197 I thought I try this library. The performance is a lot better, kudos 👏, but unfortunately it does not support the `--fineGrainedHashExternalRepos=<fineGrainedHashExternalRepos>` flag https://github.com/Tinder/bazel-diff has. Are there plans to implement this? Or is there a good alternative?