More NSDocument cleanup.

I think I'm done with that now. I moved some methods around, sadly I can't make PBGitRepository completely self-contained because there's no way to create a new document at a specific path, so `New` stuff is handled by creating a repo with `GTRepository` and then opening that. So now `GTRrepository` handles all repository creation ;-).

I tested the Recent Menu a good while — since I removed a good chunk of custom code — but haven't been able to make it behave strangely. If you can get it to do something wrong, by all means yell.

Still untested, moving around the repository folder. I'm wondering how the code'll react to that though ;-).