Use a real RT index when setting up partition tuple routing.
Before, we always used a dummy value of 1, but that's not right when
the partitioned table being modified is inside of a WITH clause
rather than part of the main query.
Amit Langote, reported and reviewd by Etsuro Fujita, with a comment
change by me.
Discussion: http://postgr.es/m/
ee12f648-8907-77b5-afc0-
2980bcb0aa37@lab.ntt.co.jp