pgsql: Use a real RT index when setting up partition tuple routing.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f81a91db4d1c2032632aa5df9fc14be24f5fe5ec

Modified Files
--------------
src/backend/commands/copy.c | 1 +
src/backend/executor/execMain.c | 3 ++-
src/backend/executor/nodeModifyTable.c | 1 +
src/include/executor/executor.h | 1 +
src/test/regress/expected/insert.out | 18 ++++++++++++++++++
src/test/regress/sql/insert.sql | 18 ++++++++++++++++++
6 files changed, 41 insertions(+), 1 deletion(-)