Given a diff as generated by record_diff, append a diff record to the
    list of diff_hist records.

def append_diff_hist(diff, diff_hist=list()):
    """Given a diff as generated by record_diff, append a diff record to the
    list of diff_hist records."""
    diff, diff_hist = _norm_json_params(diff, diff_hist)

    if not diff_hist:
        diff_hist = list()

    diff_hist.append({'diff': diff, 'diff_date': now_field()})
    return diff_hist