Fix right-anti-joins when the inner relation is proven unique

Fix right-anti-joins when the inner relation is proven unique

For an inner_unique join, we always assume that the executor will stop
scanning for matches after the first match.  Therefore, for a mergejoin
that is inner_unique and whose mergeclauses are sufficient to identify a
match, we set the skip_mark_restore flag to true, indicating that the
executor need not do mark/restore calls.  However, merge-right-anti-join
did not get this memo and continues scanning the inner side for matches
after the first match.  If there are duplicates in the outer scan, we
may incorrectly skip matching some inner tuples, which can lead to wrong
results.

Here we fix this issue by ensuring that merge-right-anti-join also
advances to next outer tuple after the first match in inner_unique
cases.  This also saves cycles by avoiding unnecessary scanning of inner
tuples after the first match.

Although hash-right-anti-join does not suffer from this wrong results
issue, we apply the same change to it as well, to help save cycles for
the same reason.

Per bug #18522 from Antti Lampinen, and bug #18526 from Feliphe Pozzer.
Back-patch to v16 where right-anti-join was introduced.

Author: Richard Guo
Discussion: https://postgr.es/m/18522-c7a8956126afdfd0@postgresql.org