ContentEntityStorageTrait uses current entity as original, which breaks things

In the course of updating Lightning's bundled version of Multiversion from alpha10 to alpha12, I discovered this bug.

Here is how it manifested, under alpha11 and later:

1. Create a new node.
2. Create a couple of new revisions of that node.
3. Visit the Revisions tab.
4. Weep bitterly as you realize that only the first revision is showing up in the list.

I chased this through the translation system and discovered what's happening here. The problem is in ContentEntityStorageTrait::doPreSave(). Specifically, this elseif clause:

    elseif (!$entity->isNew() && !isset($entity->original)) {
      $entity->original = $entity;
    }

The translation system is a terrifying labyrinth to me, but these lines are ultimately causing the entity's revision_translation_affected flag to be set incorrectly. Because of that, the Revisions tab -- which only displays revisions that changed in the current language -- is not showing everything it should.

The fault actually occurs in ContentEntityStorageBase::populateAffectedRevisionTranslations(). It calls $translation->hasTranslationChanges(), which is returning a false negative because...the original entity is the same as the updated entity, so of course there are no changes! All because that sneaky elseif clause made it so.

I'm attaching a patch which fixes this -- if $entity->originalId is set, it will load that one, unchanged. Otherwise, it'll do a standard loadUnchanged() with the current entity ID. That way, the translation system will not diff inaccurate data, and all flags will (as far as I know) be set correctly.

P.S. @timmillwood pointed me to #2597516: Remove unnecessary clone and #2792617: Issue with uuid filter, which may provide some clues as to why this breaking change was introduced in the first place.