Adjust estimate_num_groups() to not clamp per-relation group count

estimate to less than the number of values estimated for any one grouping
Var, as suggested by Manfred.  This is intuitively right, and what's
more it puts the plan choices in the subselect regression test back the
way they were before ...

src/test/regress/expected/subselect.out

@@ -134,11 +134,11 @@ SELECT '' AS five, f1 AS "Correlated Field"
                      WHERE f3 IS NOT NULL);
  five | Correlated Field 
 ------+------------------
+      |                2
       |                3
       |                1
+      |                2
       |                3
-      |                2
-      |                2
 (5 rows)
 
 --