mergejoin possibility where the inner rel was less well sorted than
the outer (ie, it matches some but not all of the merge clauses that
can work with the outer), if the inner path in question is also the
overall cheapest path for its rel.  This is an old bug, but I'm not
sure it's worth back-patching, because it's such a corner case.
Noted while investigating a test case from Peter Hardman.