pgsql: Apply my original fix for Taiki Yamaguchi's bug report about

Log Message:
-----------
Apply my original fix for Taiki Yamaguchi's bug report about DISTINCT MAX().
Add some regression tests for plausible failures in this area.

Modified Files:
--------------
    pgsql/src/backend/optimizer/path:
        equivclass.c (r1.9 -> r1.10)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/equivclass.c?r1=1.9&r2=1.10)
    pgsql/src/backend/optimizer/plan:
        planagg.c (r1.36 -> r1.37)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planagg.c?r1=1.36&r2=1.37)
    pgsql/src/include/optimizer:
        paths.h (r1.103 -> r1.104)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/paths.h?r1=1.103&r2=1.104)
    pgsql/src/test/regress/expected:
        aggregates.out (r1.16 -> r1.17)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/aggregates.out?r1=1.16&r2=1.17)
    pgsql/src/test/regress/sql:
        aggregates.sql (r1.13 -> r1.14)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/aggregates.sql?r1=1.13&r2=1.14)