pgsql: Set relispartition correctly for index partitions

Set relispartition correctly for index partitions

Oversight in commit 8b08f7d4820f: pg_class.relispartition was not
being set for index partitions, which is a bit odd, and was also causing
the code to unnecessarily call has_superclass() when simply checking the
flag was enough.

Author: Álvaro Herrera
Reported-by: Amit Langote
Discussion: https://postgr.es/m/12085bc4-0bc6-0f3a-4c43-57fe0681772b@lab.ntt.co.jp

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9e9befac4a2228ae8a5309900645ecd8ead69f53

Modified Files
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src/backend/catalog/index.c | 1 +
src/backend/commands/tablecmds.c | 51 ++++++++++++++++++++++++++++++++++------
2 files changed, 45 insertions(+), 7 deletions(-)