Submit: Use updated change for response

The response was sent with the change instance from before it
got submitted, rather than the updated change. As a result, the
response contained the status "NEW" rather than "MERGED".

Bug: Issue 12959
Change-Id: I0bbe648e7fe82dafcf55df348818ece3cb594b90

java/com/google/gerrit/server/restapi/change/Submit.java

@@ -215,7 +215,7 @@ public class Submit
       }
 
       if (updatedChange.isMerged()) {
-        return Response.ok(new Output(change));
+        return Response.ok(new Output(updatedChange));
       }
 
       String msg =