pgsql: Consider unsorted paths in generate_useful_gather_paths

Consider unsorted paths in generate_useful_gather_paths

generate_useful_gather_paths used to skip unsorted paths (without any
pathkeys), but that is unnecessary - the later code actually can handle
such paths just fine by adding a Sort node. This is clearly a thinko,
preventing construction of useful plans.

Backpatch to 13, where Incremental Sort was introduced.

Author: James Coleman
Reviewed-by: Tomas Vondra
Backpatch-through: 13
Discussion: https://postgr.es/m/CAAaqYe8cK3g5CfLC4w7bs=hC0mSksZC=H5M8LSchj5e5OxpTAg@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f4a3c0b06250ddc8ae09b59b87cf68e9bc0d7ca1

Modified Files
--------------
src/backend/optimizer/path/allpaths.c          | 11 ++---------
src/test/regress/expected/incremental_sort.out | 13 +++++++++++++
src/test/regress/sql/incremental_sort.sql      |  4 ++++
3 files changed, 19 insertions(+), 9 deletions(-)