pgsql: Fix failure of setrefs.c to process child_append_relid_sets

Fix failure of setrefs.c to process child_append_relid_sets

Commit 7358abcc6076f4b2530d10126ab379f8aea612a5 taught the planner to
store information about Append node consolidation in the final plan,
but it forgot that child_append_relid_sets needs to be adjusted
at setrefs.c time just like every other place where we store RTIs
in a plan tree. Fix that.

Although this can cause incorrect plan advice to be emitted in cases
involving multi-level partitioning, it seems more appropriate to put
the test case for it in pg_overexplain, so this commit does that.

Backpatch-through: 19
Discussion: http://postgr.es/m/CA+TgmoYYHC4i-Gpzc9-JYHqih6pOK+6LTYFiNR-PioXG1nKkOQ@mail.gmail.com

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b718dafc70a348dd7876858f5c4c42554a1cf568

Modified Files
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contrib/pg_overexplain/expected/pg_overexplain.out | 75 ++++++++++++++++++++++
contrib/pg_overexplain/sql/pg_overexplain.sql | 23 +++++++
src/backend/optimizer/plan/setrefs.c | 23 +++++++
3 files changed, 121 insertions(+)