pgsql: Fix YA incremental sort bug.

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

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/10fcb83da6a7c5328f61ca7fb60f78c57db1bd58

Modified Files
--------------
src/backend/executor/nodeIncrementalSort.c | 7 +++++++
src/test/regress/expected/incremental_sort.out | 11 +++++++++++
src/test/regress/sql/incremental_sort.sql | 3 +++
3 files changed, 21 insertions(+)