SideBySide2: Implement "No Differences"

If the content of a file is unchanged, display a red "No Differences"
prompt in the middle of the header.

Change-Id: I9397fbfb42370a6a87b32d8098dbf4f1212548f2

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/Header.java

@@ -55,6 +55,8 @@ class Header extends Composite {
   @UiField CheckBox reviewed;
   @UiField Element filePath;
 
+  @UiField Element noDiff;
+
   @UiField InlineHyperlink prev;
   @UiField InlineHyperlink up;
   @UiField InlineHyperlink next;
@@ -208,4 +210,8 @@ class Header extends Composite {
   String getNextPath() {
     return nextPath;
   }
+
+  void removeNoDiff() {
+    noDiff.removeFromParent();
+  }
 }