pg_pathman's update checker

It's necessary to check that ALTER EXTENSION pg_pathman UPDATE produces an SQL frontend that is exactly the same as a fresh install.

Usage:

PG_CONFIG=... ./dump_pathman_objects %DBNAME%

diff file_1 file_2

check_update.py script tries to verify that update is ok automatically. For instance,

tests/update/check_update.py d34a77e worktree