Re-enable persist in updateEntity()

[Pierstoval]

Even though $em->persist($entity) does nothing for entities that are already managed by the EntityManager when they're fetched from a Repository or via a join, it is still mandatory to execute persist() on it for projects that use a different tracking policy: https://www.doctrine-project.org/projects/doctrine-orm/en/current/reference/change-tracking-policies.html

My side-project Compotes uses this, and I noted a good impact in terms of performances and memory in dev (not benchmarked though) because a flush() doesn't need to check for all the entities in the memory, only the ones that were manually persisted.

And to make this work, an entity must be re-persisted if you want to update it in the database. Hence this PR.

[javiereguiluz]

Thanks for fixing this issue. Advanced Doctrine is something that I don't know well ... so thanks for explaining it in detail!