pgsql: Inherit parent's AM for partition MERGE/SPLIT operations

Inherit parent's AM for partition MERGE/SPLIT operations

This commit makes new partitions created by ALTER TABLE ... SPLIT PARTITION
and ALTER TABLE ... MERGE PARTITIONS commands inherit the paret table access
method.

Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/84ada05b-be5c-473e-6d1c-ebe5dd21b190%40gmail.com
Reviewed-by: Pavel Borisov

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/259c96fa8f78c0446eba1880850ea6fbe5092120

Modified Files
--------------
doc/src/sgml/ref/alter_table.sgml             |  2 ++
src/backend/commands/tablecmds.c              |  6 ++++++
src/test/regress/expected/partition_merge.out | 18 ++++++++++++++++++
src/test/regress/expected/partition_split.out | 23 +++++++++++++++++++++--
src/test/regress/sql/partition_merge.sql      | 13 +++++++++++++
src/test/regress/sql/partition_split.sql      | 14 ++++++++++++++
6 files changed, 74 insertions(+), 2 deletions(-)