Bug 1215617 - Print a friendly message when compare view is empty

Including a hint to change the filters.

ui/partials/perf/comparetable.html

@@ -94,3 +94,9 @@
     </tr>
   </tbody>
 </table>
+<p class="lead text-center" ng-show="!filteredTestList.length">
+  No results to show for these two revisions.
+  <span ng-show="testList.length">
+    Try changing your filter settings?
+  </span>
+</p>