Add support for OpenDream testmerges

[ike709]

Now that TGS supports OpenDream, OD would benefit greatly from being able to do testmerges of OD itself rather than just the SS13 code that it's running.

I think that just being able to add an open pull request as an engine version installation would be sufficient.

[Cyberboss]

Just need to break up the functionality of IRepository.AddTestMerge into a fetch step and then that can be called for PRs.