GitReferenceUpdated: Add helper methods to get update type

The type of ref update is already known on the server before the ref
update event is fired. Pass this information into the event, so its
consumers can get it without having to perform operations on the git
repository.

Change-Id: I5d90a157c316ac0c2b65ba3deb2cc2f62628df8a

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/events/GitReferenceUpdatedListener.java

@@ -25,6 +25,9 @@ public interface GitReferenceUpdatedListener {
     String getRefName();
     String getOldObjectId();
     String getNewObjectId();
+    boolean isCreate();
+    boolean isDelete();
+    boolean isNonFastForward();
   }
 
   void onGitReferenceUpdated(Event event);