#110857 by rotzi (with some changes by dww): complete re-write of the

module. Now based on the GPL'ed mediawiki PHP diff engine from phpwiki.
Instead of a separate diff tab, this overrides the revisions tab and
provides radio buttons to select which revisions to view diffs between.
Shows diffs in unfiltered, raw node data, instead of rendered HTML.
Also, provides a hook_diff() for modules to define any parts of a node
that they control with revisions (e.g. file uploads, etc) for display.