pgsql: Teach convert_subquery_pathkeys() to handle the case where the

Log Message:
-----------
Teach convert_subquery_pathkeys() to handle the case where the
subquery's pathkey is a RelabelType applied to something that appears
in the subquery's output; for example where the subquery returns a
varchar Var and the sort order is shown as that Var coerced to text.
This comes up because varchar doesn't have its own sort operator.
Per example from Peter Hardman.

Modified Files:
--------------
    pgsql/src/backend/optimizer/path:
        pathkeys.c (r1.77 -> r1.78)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/pathkeys.c.diff?r1=1.77&r2=1.78)