Make `krel fast-forward` work for other repositories like k/website

There is a sync job in place which fast-forwards the kubernetes/kubernetes `release-1.x` branch from `master` until the final (non-prerelease) tag exists on it. The job runs periodically in prow and does basically nothing when we're not in code freeze: https://testgrid.k8s.io/sig-release-releng-blocking#git-repo-kubernetes-fast-forward

The idea is to re-use the logic for other repositories like `k/website` to sync the documentation development branch during the release cycle. We have to make the tool more flexible from a CLI perspective and double check the code paths for possible adaptations.

The entrypoint for the code of `krel fast-forward` is there: https://github.com/kubernetes/release/blob/master/cmd/krel/cmd/ff.go