New RPC to check whether remote name exists
To fix https://gitlab.com/gitlab-org/gitlab-ce/issues/50562, we would like to check if the remote name exists on a repository before adding it unless we are okay with calling
RemoteService#AddRemote and letting
git complain with
fatal: remote my-remote already exists.
I notice we have
RemoteService#RemoteRepositoryExists, but this actually just checks whether the remote URL exists. I thought it would behave similarly to
RemoteService#RemoteRemote, but it does not. I think we want to add