Problem with zero year

We added the following commit in 8.4:
/src/backend/utils/adt/datetime.c                                                                      tgl Reject year
zeroduring datetime input, except when it's a 2-digit year (then it means 2000 AD).  Formerly we silently interpreted
thisas 1 BC, which at best is unwarranted familiarity with the implementation. It's barely possible that some app
somewhereexpects the old behavior, though, so we won't back-patch this into existing release branches.
 

The problem is that the 2-digit year check is for <=2 digits, not
exactly two digits:
   /*    * When processing a year field, mark it for adjustment if it's only one    * or two digits.    */   if (*tmask
==DTK_M(YEAR))       *is2digits = (flen <= 2);
 

This leads to some unexpected outputs:
test=> select '1-1-0'::date;   date------------ 2000-01-01
test=> select '1-1-0 BC'::date;ERROR:  date/time field value out of range: "1-1-0 BC"LINE 1: select '1-1-0 BC'::date;
           ^
 
test=> select '1-1-0 AD'::date;    date------------ 2000-01-01
test=> select '1-1-000'::date;ERROR:  date/time field value out of range: "1-1-000"LINE 1: select '1-1-000'::date;

I think the BC part is fine because that can't possibily be 2000 AD, but
having '0' interpreted as 2000 seems counter to the commit message text;
should the assignment be changed to:
   if (*tmask == DTK_M(YEAR))       *is2digits = (flen == 2);

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