pgsql: Teach nodeMergejoin how to handle DESC and/or NULLS FIRST sort

Log Message:
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Teach nodeMergejoin how to handle DESC and/or NULLS FIRST sort orders.
So far only tested by hacking the planner ...

Modified Files:
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pgsql/src/backend/executor:
nodeMergejoin.c (r1.85 -> r1.86)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeMergejoin.c.diff?r1=1.85&r2=1.86)