Ah, restored altered dirname code to Monotone's root method.

Ah, restored altered dirname code to Monotone's root method.

The previous implementation used
  cwd=path
which would fail for non-directory paths.

The implementation before that was missing the not from
  if not os.path.isdir(path):
      dirname = os.path.dirname(path)
which meant it found the dirname when it didn't need to, and not when
it did ;).