pgsql: Fix ALTER TABLE .. ADD COLUMN with complex inheritance trees

Fix ALTER TABLE .. ADD COLUMN with complex inheritance trees

This command, when used to add a column on a parent table with a complex
inheritance tree, tried to update multiple times the same tuple in
pg_attribute for a child table when incrementing attinhcount, causing
failures with "tuple already updated by self" because of a missing
CommandCounterIncrement() between two updates.

This exists for a rather long time, so backpatch all the way down.

Reported-by: Alexander Lakhin
Author: Tender Wang
Reviewed-by: Richard Guo
Discussion: https://postgr.es/m/18297-b04cd83a55b51e35@postgresql.org
Backpatch-through: 12

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/5a7833f49679a3e8a17df453911b0adacea6273d

Modified Files
--------------
src/backend/commands/tablecmds.c | 4 ++++
src/test/regress/expected/inherit.out | 17 +++++++++++++++++
src/test/regress/sql/inherit.sql | 9 +++++++++
3 files changed, 30 insertions(+)