PBGitRepository: Fix opening of large directories due to bug in NSFileWrapper.
This commit changes readFromFileWrapper:ofType:error: to readFromURL:ofType:error:.
The default implementation of readFromURL calls readFromFileWrapper, but a bug
in NSFileWrapper makes this fail with repositories with a large number of files
(not sure if the fact that those files were in subdirectories matter or not).
So instead we skip the whole FileWrapper thing, which we don't really use anyway,
and just use the URL.