Fix remote mirrors failing if Git remotes have not been added

Remote mirrors only get created when the URL changes, However, during the GCP migration, the remote mirror did not get created automatically. Plus, there's no guarantee someone restoring a repository from backup would have this remote. We now add the remote each time we attempt to fetch from the repository.

This works because Gitaly doesn't throw up an exception or error if the remote already exists: gitaly#1317 (closed)

In the future, we should attempt to add if the remote doesn't exist: gitaly#1316 (closed)

Closes #50562 (closed)