pg_upgrade rebuild_tsvector_tables.sql includes child table columns

When pg_upgrade generates a rebuild_tsvector_tables.sql script to
rewrite tsvector columns, it includes ALTER TABLE commands for child
tables that cannot be altered independently from their parents:

psql:rebuild_tsvector_tables.sql:673: ERROR: cannot alter inherited column "xxx"

This isn't a problem unless you run the script in single-transaction
mode, because the commands will just fail and the work has already
been done in the parent table.

It's probably not worth fixing this, given that this only applies to
upgrades from 8.3, but I wanted to get it into the archives. If we
ever need to do something like this again, we should make it smarter.