pgsql: Fix planning of SubLinks to ensure that Vars generated from

Log Message:
-----------
Fix planning of SubLinks to ensure that Vars generated from transformation of
a sublink's test expression have the correct vartypmod, rather than defaulting
to -1.  There's at least one place where this is important because we're
expecting these Vars to be exactly equal() to those appearing in the subplan
itself.  This is a pretty klugy solution --- it would likely be cleaner to
change Param nodes to include a typmod field --- but we can't do that in the
already-released 8.2 branch.
Per bug report from Hubert Fongarnand.

Modified Files:
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    pgsql/src/backend/optimizer/plan:
        subselect.c (r1.112 -> r1.113)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/subselect.c.diff?r1=1.112&r2=1.113)