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

[saschagrunert]

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

[saschagrunert]

@cpanato volunteered to work on this

/assign @cpanato

[cpanato]

this will require some code changes I will start working on this part

[cpanato]

not yet done
/reopen

[k8s-ci-robot]

@cpanato: Reopened this issue.

In response to this:

not yet done
/reopen

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