Run fake fchown code if fileno points to a sub-path of our tree.

        The ownership set with this fake fchown can be inspected by looking
        at the self.uid/self.gid dictionaries.

def _fchown(self, real, fileno, uid, gid):
        """Run fake fchown code if fileno points to a sub-path of our tree.

        The ownership set with this fake fchown can be inspected by looking
        at the self.uid/self.gid dictionaries.
        """
        path = self._fake_path(self._path_from_fd(fileno))
        self._chown_common(path, uid, gid)