a8n: Align ChangsetPlan/ExternalChangset interfaces
Created by: mrnugget
This fixes #6752 (closed) by reintroducing the PreviewRepositoryComparison (previously called: PreviewRepositoryDiff) interface on ChangsetPlan.
The campaignJobResolver implements this interface without having the need to define another resolver.
@eseliger: Let me know if that's what you had in mind.