Update generated diff to match ORM schema diff

[cristi-contiu]

Q	A
Type	improvement
BC Break	no/minor
Fixed issues	#1487

Summary

Ports doctrine/orm#7875 into Migrations to improve consistency between queries generated by Migrations diff and the ones generated by the ORM schema update/validate commands. In most cases, the queries where already identical, but in some rare cases (like #1487), the migrations generated from diff did not contain the same queries as the ORM schema update and validate commands.

It also removes filtering the schema generated from metadata ($toSchema) using DBAL's schemaAssetsFilter and the doctrine.dbal.schema_filter config, which should only be used to exclude tables in the schema generated from the existing database ($fromSchema - DBAL and DoctrineBundle already cover this). This might be considered a minor BC-break, but if the metadata contains unwanted tables, they shouldn't be included in the mappings in the first place.

The decoration of the existing schemaAssetsFilter from the DBAL configuration is done in doctrine/orm#7875 to whitelist assets present in the $toSchema so that they don't get removed from the $fromSchema by the default schemaAssetsFilter (usually based on the doctrine.dbal.schema_filter config) - a difference which would lead to always generating CREATE TABLE queries.

I think this last part is best tested in a functional/integration test, but I would need some guidance on how to set it up.

If it's a wanted change, I can also add a note to UPGRADE.md, depending on the branch it would be included in and if it's considered a minor BC break.

[cristi-contiu]

Accidentaly closed this PR, opened a new one here: #1490