Worktree support

[tiennou]

I've had to use a worktree-ed repo, and I'm a GitX user, which uses Objective-Git.

This is a complete work-in-progress, because I just wanted to be able to run against worktrees, so even though I tried to map some of the API, it's completely untested (GitX doesn't even call anything in there yet).

Since most of the wrapping is done, I'm putting that there. Maybe I'll finish it sometime (maybe not), or hopefully someone can pick it up and write tests for it 😉.