pgsql: Fix wrong week returnded by date_trunc('week') for early dates in

[momjian(at)svr1(dot)postgresql(dot)org (Bruce Momjian)]

Log Message:
-----------
Fix wrong week returnded by date_trunc('week') for early dates in
January --- would return wrong year for 2005-01-01 and 2006-01-01.

per report from Robert Creager.

Backpatch to 8.0.X.

Tags:
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REL8_0_STABLE

Modified Files:
--------------
    pgsql/doc/src/sgml:
        func.sgml (r1.234.4.4 -> r1.234.4.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.234.4.4&r2=1.234.4.5)
    pgsql/src/backend/utils/adt:
        timestamp.c (r1.117 -> r1.117.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/timestamp.c.diff?r1=1.117&r2=1.117.4.1)