Return the build/revision date as an integer "yyyymmdd".

def _dateversion(self):
        # type: () -> int
        """Return the build/revision date as an integer "yyyymmdd"."""
        import re
        if self._head:
            ma = re.search(r'(?<=\()(.*)(?=\))', self._head)
            if ma:
                s = re.split(r'[, ]+', ma.group(0))
                if len(s) >= 3:
                    # month
                    month_names = ('Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
                                   'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec')
                    if s[0] in month_names:
                        m = month_names.index(s[0]) + 1
                        # date
                        if s[1].isdigit():
                            d = int(s[1])
                            if 1 <= d <= 31:
                                # year
                                if s[2].isdigit():
                                    y = int(s[2])
                                    if y >= 1:
                                        # Return an integer as "yyyymmdd".
                                        return y * 10000 + m * 100 + d
            raise ValueError('failed to parse "{0}"'.format(self._head))
        raise ValueError('no first line')