[PolyGerrit] Render (read-only) comments and column width ruler.

Also some small additional changes as well:
+ Correct some :host selectors that were causing issues when Shadow DOM
  was used instead of Shady DOM.
+ Two of the same revisions should not be shown in side-by-side view.
  When two are specified via the url as /N..N/ redirect to /N/.
+ Added header to the diff view to give context and allow the user to
  go back to the top-level change.

Feature: Issue 3676
Feature: Issue 3648
Feature: Issue 3649
Change-Id: Ib39bb7cb2eab26740f80971d4e00d579a2e08b0b

polygerrit-ui/app/scripts/app.js

@@ -53,6 +53,12 @@
         patchNum: ctx.params[4],
         path: ctx.params[5]
       };
+      // Don't allow diffing the same patch number against itself because WHY?
+      if (params.basePatchNum == params.patchNum) {
+        page.redirect('/c/' + params.changeNum + '/' + params.patchNum + '/' +
+            params.path);
+        return;
+      }
       if (!params.patchNum) {
         params.patchNum = params.basePatchNum;
         delete(params.basePatchNum);