Support LogicalRepeatUnion in RelShuttle

@Test void testRelShuttleForLogicalRepeatUnion() {
  final String sql = "WITH RECURSIVE delta(n) AS (\n"
      + "VALUES (1)\n"
      + "UNION ALL\n"
      + "SELECT n+1 FROM delta WHERE n < 10\n"
      + ")\n"
      + "SELECT * FROM delta";
  final RelNode rel = sql(sql).toRel();
  final List<RelNode> rels = new ArrayList<>();
  final RelShuttleImpl visitor = new RelShuttleImpl() {
    @Override public RelNode visit(LogicalRepeatUnion repeatUnion) {
      RelNode visitedRel = super.visit(repeatUnion);
      rels.add(visitedRel);
      return visitedRel;
    }
  };
  rel.accept(visitor);
  assertThat(rels, hasSize(1));
  assertThat(rels.get(0), instanceOf(LogicalRepeatUnion.class));
}