pgsql: Do not allow Unique nodes to be scanned backwards.

Log Message:
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Do not allow Unique nodes to be scanned backwards.  The code claimed that it
would work, but in fact it didn't return the same rows when moving backwards
as when moving forwards.  This would have no visible effect in a DISTINCT
query (at least assuming the column datatypes use a strong definition of
equality), but it gave entirely wrong answers for DISTINCT ON queries.

Tags:
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REL8_1_STABLE

Modified Files:
--------------
    pgsql/src/backend/executor:
        execAmi.c (r1.85.2.2 -> r1.85.2.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execAmi.c?r1=1.85.2.2&r2=1.85.2.3)