pgsql: Copy partitioned_rels lists to avoid shared substructure.

Copy partitioned_rels lists to avoid shared substructure.

Otherwise, set_plan_refs() can get applied to the same list
multiple times through different references, leading to chaos.

Amit Langote, Dilip Kumar, and Robert Haas, reviewed by Ashutosh
Bapat. Original report by Sveinn Sveinsson.

Discussion: http://postgr.es/m/20170517141151.1435.79890@wrigleys.postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b522759508dae17535f8cd20598a50a409a97f4d

Modified Files
--------------
src/backend/optimizer/util/pathnode.c | 6 +++---
src/test/regress/expected/inherit.out | 31 +++++++++++++++++++++++++++++++
src/test/regress/sql/inherit.sql | 10 ++++++++++
3 files changed, 44 insertions(+), 3 deletions(-)