Apply the "LIMIT 1" optimization to partial DISTINCT

In 5543677ec9 we introduced an optimization that uses Limit instead of
Unique to implement DISTINCT when all the DISTINCT pathkeys have been
marked as redundant.  I happened to notice that this optimization was
not applied to partial DISTINCT, which I think should be.  This can
improve plans in some cases, such as

-- on master
explain (costs off) select distinct four from tenk1 where four = 4;
                  QUERY PLAN
----------------------------------------------
 Limit
   ->  Gather
         Workers Planned: 4
         ->  Unique
               ->  Parallel Seq Scan on tenk1
                     Filter: (four = 4)
(6 rows)

-- patched
explain (costs off) select distinct four from tenk1 where four = 4;
                  QUERY PLAN
----------------------------------------------
 Limit
   ->  Gather
         Workers Planned: 4
         ->  Limit
               ->  Parallel Seq Scan on tenk1
                     Filter: (four = 4)
(6 rows)

Such queries might not be that common, but it's very cheap to apply this
optimization.

Attached is a patch for that.

Thanks
Richard