Improve diff refs and factories

What does this MR do?

This adds some improvements to diff refs and their factories discovered during work on #13495 (closed)

Specifically:

• methods are changed to return predicates
• factories are introduced where they were previously absent

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Fixes #42741 (closed)