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