Fix the miss consideration of tuple_fraction during add_paths_to_append_rel

When I am working on "Pushing limit into subqueries of a union" [1], I
found we already have a great infrastructure to support this. For a query
like

subquery-1 UNION ALL subquery-2 LIMIT 3;

We have considered the root->tuple_fraction when planning the subqueries
without an extra Limit node as an overhead. But the reason it doesn't work
in my real case is flatten_simple_union_all flat the union all subqueries
into append relation and we didn't handle the root->tuple_fraction during
add_paths_to_append_rel.

Given the below query for example:
explain analyze
(select * from tenk1 order by hundred)
union all
(select * from tenk2 order by hundred)
limit 3;

Without the patch: Execution Time: 7.856 ms
with the patch: Execution Time: 0.224 ms

Any suggestion is welcome.

[1] https://www.postgresql.org/message-id/11228.1118365833%40sss.pgh.pa.us

--
Best Regards
Andy Fan