Allow any two versions of any two pages to be diffed and the output shown inline in a page.

One version old to current version of ~SomePage {{[[{Diff from='~SomePage@-1' to='~SomePage'}]}}

Version 12 to Version 46 of ~SomePage {{[[{Diff from='~SomePage@12' to='~SomePage@46'}]}}

Between current revisions of __different__ pages {{[[{Diff from='~DraftSpecifications' to='~FrozenSpecifications'}]}}

And so on...

Of course this should use the configured DiffProvider, or allow it to be specified...