Useless override

This is called by -readFromURL:ofType:error, but we're overriding that, so...
Use our ivar instead of allocating a new copy
Get our HEAD detached status directly from our GTRepository ivar
Remove things that aren't needed now that PBGitRepository is more NSDoc-like

Also, switch repository creation to GTRepository.
Add pull request #198 to the change log
Merge branch 'issues/198'