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:
https://gitlab.com/gitlab-org/gitaly/issues/1317

In the future, we should attempt to add if the remote doesn't exist:
https://gitlab.com/gitlab-org/gitaly/issues/1316

Closes #50562

This change makes closes another migration! And remotes tests around it,
mostly due to stubbing this was needed. Also, these are tested on the
Gitaly side too:
- https://gitlab.com/gitlab-org/gitaly/issues/791
-
https://gitlab.com/gitlab-org/gitaly/blob/6dd74543f6af8ebcf22aa10e39da004f388fdda5/internal/service/repository/fetch_remote_test.go

Closes https://gitlab.com/gitlab-org/gitaly/issues/789