Do not show update bar when there are no new messages

It is shown when adding a reviewer.

Change-Id: I974ad287dd7dc5f073e566757def3ca6a7d5cfd9
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/change/ChangeScreen.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/change/ChangeScreen.java
index 0f05ff2..5707a7e 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/change/ChangeScreen.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/change/ChangeScreen.java
@@ -1446,6 +1446,10 @@
       nm = JsArray.createArray().cast();
     }
 
+    if (om.length() == nm.length()) {
+      return;
+    }
+
     if (updateAvailable == null) {
       updateAvailable = new UpdateAvailableBar() {
         @Override