Add alternate comparison types to versions

- The types are:
-- Previous: The default and the previously used type
-- Subsequent: Compares against the next version
-- Current: Compares against the current version
- Allow switching between comparison types in index and diff views
-- Have links vary depending upon current comparison type

app/controllers/wiki_page_versions_controller.rb

@@ -3,6 +3,7 @@ class WikiPageVersionsController < ApplicationController
   layout "sidebar"
 
   def index
+    set_version_comparison
     @wiki_page_versions = WikiPageVersion.paginated_search(params)
     @wiki_page_versions = @wiki_page_versions.includes(:updater) if request.format.html?
 
@@ -16,15 +17,20 @@ class WikiPageVersionsController < ApplicationController
 
   def diff
     if params[:thispage].blank? || params[:otherpage].blank?
-      redirect_back fallback_location: wiki_pages_path, notice: "You must select two versions to diff"
-      return
-    end
-
-    @thispage = WikiPageVersion.find(params[:thispage])
-    @otherpage = WikiPageVersion.find(params[:otherpage])
-
-    if @thispage.id < @otherpage.id
-      @thispage, @otherpage = @otherpage, @thispage
+      page_id = params[:thispage] || params[:otherpage]
+      if page_id.blank?
+        redirect_back fallback_location: wiki_pages_path, notice: "You must select at least one version to diff"
+        return
+      end
+      set_version_comparison
+      @thispage = WikiPageVersion.find(page_id)
+      @otherpage = @thispage.send(params[:type])
+    else
+      @thispage = WikiPageVersion.find(params[:thispage])
+      @otherpage = WikiPageVersion.find(params[:otherpage])
+      if @thispage.id < @otherpage.id
+        @thispage, @otherpage = @otherpage, @thispage
+      end
     end
 
     respond_with([@thispage, @otherpage])