Replace icon for merge commit

The new icon now shows a merge graph rather than a '!'.

Users might find the '!' scary and may think that they have done
something totally wrong.

The merge icon is based on the important icon from the Tango Icon
Library [1]

[1] http://tango.freedesktop.org/Tango_Icon_Library

Change-Id: I59904b894df9074444df8a7810f1f224a7e2f6aa
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-gwtui/src/main/java/com/google/gerrit/client/GerritResources.java

@@ -70,6 +70,6 @@ public interface GerritResources extends ClientBundle {
   @Source("warning.png")
   public ImageResource warning();
 
-  @Source("important.png")
-  public ImageResource important();
+  @Source("merge.png")
+  public ImageResource merge();
 }

gerrit-gwtui/src/main/java/com/google/gerrit/client/change/CommitBox.ui.xml

@@ -128,7 +128,7 @@ limitations under the License.
           </div>
           <g:Image
               ui:field='mergeCommit'
-              resource='{ico.important}'
+              resource='{ico.merge}'
               visible='false'
               title='Merge Commit'>
             <ui:attribute name='title'/>