pgsql: Reject year zero during datetime input, except when it's a

Log Message:
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Reject year zero during datetime input, except when it's a 2-digit year
(then it means 2000 AD).  Formerly we silently interpreted this as 1 BC,
which at best is unwarranted familiarity with the implementation.
It's barely possible that some app somewhere expects the old behavior,
though, so we won't back-patch this into existing release branches.

Modified Files:
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    pgsql/src/backend/utils/adt:
        datetime.c (r1.186 -> r1.187)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c?r1=1.186&r2=1.187)