'Create a version 2 index file for this data file.
:param filename: Index filename.
:param progress: Progress report function
:return: Checksum of index file'

def create_index_v2(self, filename, progress=None):

    entries = self.sorted_entries(progress=progress)
    with GitFile(filename, 'wb') as f:
        return write_pack_index_v2(f, entries, self.calculate_checksum())
