Push to a mirror if it's defined

The use case is that we have a central git server with a webinterface.
The easiest solution is to make one push to the other and disallow
commits from anyone on the mirror. Since we have multiple smart proxies
and only a single git server I think the easiest solution is to make
each smart proxy push instead of the other way around.

By using a non-standard remote name the chance of breaking an existing
setup is very small. The user is assumed to set up the remote and
authentication.