Dump COMMENT ON SCHEMA public.

Dump COMMENT ON SCHEMA public.

As we do for other attributes of the public schema, omit the COMMENT
command when its payload would match what initdb had installed.  For
dumps that do carry this new COMMENT command, non-superusers restoring
them are likely to get an error.

Reviewed by Asif Rehman.

Discussion: https://postgr.es/m/ab48a34c-60f6-e388-502a-3e5fe46a2dae@postgresfriends.org