Missing rev1 or rev2 parameter behavior is broken

To generate a diff about a version, we currently need to write this code:

      ## Diff URL
      #set ($docAtVersion = $xwiki.getDocument($someDoc, $someVersion))
      #set ($diffUrl = $xwiki.getURL($someDoc, 'view', "viewer=changes&rev1=${docAtVersion.previousVersion}&rev2=${someVersion}"))

We need to load the document at the given version to know what is the previous version number. It won't help make xwiki faster every time you need to generate a link to have such a diff.

vmassol We could have a URL that says "compare this revision with the one before" (if we don't have that already).

Then I propose to introduce a "toRev" parameter that generates the diff between "rev - 1" and "rev".