After a revision wikitext is hidden by Special:Revisiondelete,
diff still shows its text, fetched from diff cache.
- Shows a diff title=Foo&diff=2&oldid=1 ... diff is cached as "wikiname:diff:oldid:1:newid:1"
- Hide text of revision 1 using Special:Revisiondelete.
- Shows a diff title=Foo&diff=2&oldid=1 again. ... DifferenceEngine#getDiffBody fetches the cache and shows it. ... diff contains hidden text.