Selective fetching of refs from remote repositories for mirroring

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Currently, our pull-mirrors fetch everything from the remote repository, then we iterate through all of the tags and branches to see if we need to update a local ref.

But in some cases, we don't need to fetch everything:

1. When only mirroring protected branches
2. When updating branches for running pipelines for external repositories

We could add a refspec argument to the FetchRemote RPC to allow fetching and processing only what we need?