Turn EntityComparisonBase into a service

I've made good progress with moving most of the controller logic into a service. Patch to follow in a few days.

Patch coming on Monday. Code 99.9% ready.

Having a lot of trouble getting a clean patch.

Test fails were caused by unintended removal of nonBreakingSpace property from EntityComparisonBase class

Sorry about that. I've fixed the // spaces and dots in the controllers, and some other CS issue in EntityComparisonBase. I couldn't get codesniffer to run on the controllers though, it generated some exception. However the patch touches so few lines in the controllers that any other CS issues are almost certainly pre-existing.

Question: is there any point to keeping EntityComparisonBase.php if all of its logic is moved into the service, and we only have one controller extending it? All it would do is invoke the service and make that available to controllers that extend it, but it's easy enough for the controllers to invoke the service themselves.
I think I should remove EntityComparisonBase in the next version of this patch?

I also need to wait for #2768371: Make Diff Plugins property aware as that is changing EntityComparisonBase and the controllers.

I'm trying to figure out how to give the calling context (e.g. the controller) maximum control of the result. This allows people to build diffs in different contexts with modifications to the standard display.

The logic that decides which fields to show, what diff builder to use, and then builds the diff, (all in entityParser and diffBuilderManager) happens as part of a single recursive process, walking through the tree of fields and fields on referenced entities. We don't first do a setup, deciding what fields to compare and what builders to use, and then secondly build the comparisons. Therefore the calling context can't inspect the setup before the build, and override the choice of plugin or the configured settings on a plugin or anything else.

We could split it up, but that would require walking the fields tree twice. First we would build up a setup array storing what we planned to compare, loading the configured builders and settings, and return that array to the controller (which could modify it how it wanted). Second the controller would call a different method that went through the setup array and built the comparisons.

Is there an easier way?

OK, new patch. Restarted from scratch so no interdiff.

A very simple approach: All the methods from EntityComparisonBase went up into a DiffEntityComparison service, a couple of simple properties went down into GenericRevisionContoller, and EntityComparisonBase was removed.

Let's see what testbot says.

It would help if I actually included DiffEntityComparison.php in the patch ...

For #25 I've created #2784685: Streamline table header / row in GenericRevisionController as a follow-up, because a) those lines are just copied from the existing code, and (b) I'm not quite sure how to fix it. Is that acceptable?