Set up gitolite <--> gitlab mirrors

Since our branch name change in #6 (closed), we'll have to update our mirrors and we might as well be consistent this time.

I'm proposing a one way mirror from Gitlab to git.tpo because it means we can use the gitlab merge feature. This would make git.tpo mostly read only except for the repositories that have not yet been migrated to gitlab.

However, I'm open to feedback. My only strong preference is that we're consistent.