Check for truncated output since grabix doesn't write to a transactional directory.

def _is_partial_index(gbi_file):
    """Check for truncated output since grabix doesn't write to a transactional directory.
    """
    with open(gbi_file) as in_handle:
        for i, _ in enumerate(in_handle):
            if i > 2:
                return False
    return True