Currently if a user or bot wants to compare two items or revisions on wikidata they can use the standard 'compare' api provided by Mediawiki.
For wikidata changes this provides something like the below
http://www.wikidata.org/w/api.php?action=compare&fromrev=115242402&torev=214926609 (For creating a redirect)
http://www.wikidata.org/w/api.php?action=compare&fromrev=35441647&torev=115234261 (For adding a sitelink)

The Datamodel already has lots of ways to compare versions of entities and create diff objects
can be found at https://github.com/wmde/WikibaseDataModel/tree/master/src/Entity/Diff

I propose that this be revealed through an api, wbcompare perhaps, with the same parameters as the mediawiki compare module