Filter branches for mirrored repositories

When setting up a mirrored repository, it would be nice to be able to filter which branches are mirrored; the UI could be similar to the Protected Branches UI, which allows choosing specific branches or wildcards.

My use case for this is that we have two GitLab servers, one that is for internal use only and another external one that is accessible by clients. We have repositories on the internal server that are mirrored to the external one in order to share the code with clients, but we want to have some amount of control over exactly what becomes visible to clients.

Automatic mirroring is convenient, but we don't necessarily want every single commit to the repository to automatically go to the internal one; we want developers to be able to do their work in branches, have it reviewed through merge requests to the master branch, and have only the master branch be mirrored in order to ensure that all code that is available to clients has been reviewed.