Pick the most recent revision date.

        `r_stmts` is a list of 'revision' statements.

def current_revision(self, r_stmts):
        """Pick the most recent revision date.

        `r_stmts` is a list of 'revision' statements.
        """
        cur = max([[int(p) for p in r.arg.split("-")] for r in r_stmts])
        return "%4d-%02d-%02d" % tuple(cur)