This commit implements psql tab completion for ALTER TABLE ... SPLIT PARTITION
and ALTER TABLE ... MERGE PARTITIONS commands.
Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/5dee3937-8e9f-cca4-11fb-737709a92b37%40gmail.com
Author: Dagfinn Ilmari Mannsåker, Pavel Borisov