Extracts data from a local git repository. Mutates _data.
        :param str source: The path to a local git repository.
        :param args: Arbitrary arguments for extensibility.
        :param kwargs: Arbitrary keyword arguments for extensibility.

        :return: None

def _extract(self, source, *args, **kwargs):
        """
        Extracts data from a local git repository. Mutates _data.
        :param str source: The path to a local git repository.
        :param args: Arbitrary arguments for extensibility.
        :param kwargs: Arbitrary keyword arguments for extensibility.

        :return: None
        """
        # Extract git test_data
        self._data = extract_log(source)

        # Shorten hashes
        self._data['hexsha'] = self._data['hexsha'].apply(lambda s: s[:7])