Fix YA incremental sort bug.

switchToPresortedPrefixMode() did the wrong thing if it detected
a batch boundary just at the last tuple of a fullsort group.

The initially-reported symptom was a "retrieved too many tuples in a
bounded sort" error, but the test case added here just silently gives
the wrong answer without this patch.

I (tgl) am not really happy about committing this patch without review
from the incremental-sort authors, but they seem AWOL and we are hard
against a release deadline.  This does demonstrably make some cases
better, anyway.

Per bug #16846 from Yoran Heling.  Back-patch to v13 where incremental
sort was introduced.

Neil Chen

Discussion: https://postgr.es/m/16846-ae49f51ac379a4cb@postgresql.org

src/backend/executor/nodeIncrementalSort.c

@@ -394,6 +394,13 @@ switchToPresortedPrefixMode(PlanState *pstate)
 				 * current prefix key group.
 				 */
 				ExecClearTuple(node->group_pivot);
+
+				/*
+				 * Also make sure we take the didn't-consume-all-the-tuples
+				 * path below, even if this happened to be the last tuple of
+				 * the batch.
+				 */
+				lastTuple = false;
 				break;
 			}
 		}