Migrations are executed in transactions anyway, and creating a savepoint can cause
spurious failures on databases that don't support transactional DDL (like
MySQL and MariaDB) when it attempts to commit a savepoint that was silently
not created because there wasn't an active transaction after some DDL was
executed.
While a better solution might involve splitting this migration into several so each
one is only DDL or only data manipulation, I don't think that can be done very
easily while maintaining compatibility with existing deployments.
Fixes#3229.
db64f54010