pgsql: Set ecxt_scantuple correctly for tuple routing.

Set ecxt_scantuple correctly for tuple routing.

In 2ac3ef7a01df859c62d0a02333b646d65eaec5ff, we changed things so that
it's possible for a different TupleTableSlot to be used for partitioned
tables at successively lower levels. If we do end up changing the slot
from the original, we must update ecxt_scantuple to point to the new one
for partition key of the tuple to be computed correctly.

Reported by Rajkumar Raghuwanshi. Patch by Amit Langote.

Discussion: http://postgr.es/m/CAKcux6%3Dm1qyqB2k6cjniuMMrYXb75O-MB4qGQMu8zg-iGGLjDw%40mail.gmail.com

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/132488bfee687865375b5410f18a78fb55bd7015

Modified Files
--------------
src/backend/catalog/partition.c | 30 +++++++++++++++++++++++-------
src/backend/executor/execMain.c | 2 --
src/test/regress/expected/insert.out | 2 +-
src/test/regress/sql/insert.sql | 2 +-
4 files changed, 25 insertions(+), 11 deletions(-)