Add label for no changes revision

Author: takezoe

src/main/twirl/gist/revisions.scala.html

@@ -13,6 +13,9 @@
           <div style="margin-bottom: 8px;">
             @avatar(revision, 20) <a href="@path/gist/@revision.authorName">@revision.authorName</a> revised this @helper.html.datetimeago(revision.authorTime)
             <div class="pull-right">
+              @if(diffs.isEmpty){
+                <span class="muted">No changes.</span>
+              }
               <a href="@path/gist/@gist.userName/@gist.repositoryName/@revision.id" class="btn btn-mini monospace">@revision.id.substring(0, 7)</a>
             </div>
           </div>