Include genrule cmd in the metahash.

def _metahash(self):
        """Include genrule cmd in the metahash."""
        if self._cached_metahash:
            return self._cached_metahash
        mhash = base.BaseBuilder._metahash(self)
        log.debug('[%s]: Metahash input: cmd="%s"', self.address, self.cmd)
        mhash.update(self.cmd)
        self._cached_metahash = mhash
        return mhash