Consider explicit incremental sort for Append and MergeAppend

For ordered Append or MergeAppend, it seems that incremental sort is
currently not considered when injecting an explicit sort into subpaths
that are not sufficiently ordered. For instance:

set enable_seqscan to off;

explain (costs off)
select hundred as x, two as y from tenk1
union all
select thousand as x, tenthous as y from tenk1
order by x, y;
QUERY PLAN
-------------------------------------------------------------------
Merge Append
Sort Key: tenk1.hundred, tenk1.two
-> Sort
Sort Key: tenk1.hundred, tenk1.two
-> Index Scan using tenk1_hundred on tenk1
-> Index Only Scan using tenk1_thous_tenthous on tenk1 tenk1_1
(6 rows)

Similar to what we do in 828e94c9d, I think we can also consider using
explicit incremental sort for ordered Append or MergeAppend. Here is
a patch doing that. With this patch, the plan above changes to:

QUERY PLAN
-------------------------------------------------------------------
Merge Append
Sort Key: tenk1.hundred, tenk1.two
-> Incremental Sort
Sort Key: tenk1.hundred, tenk1.two
Presorted Key: tenk1.hundred
-> Index Scan using tenk1_hundred on tenk1
-> Index Only Scan using tenk1_thous_tenthous on tenk1 tenk1_1
(7 rows)

This targets v19.

Thanks
Richard