actually write the diff event

Author: baltzell

common-tools/clas-io/src/main/java/org/jlab/utils/HipoDiff.java

@@ -185,7 +185,7 @@ public static int compare(Event a, Event b) {
             if (writer != null) {
                 event.reset();
                 event.write(banksA.get(i).getDiff(banksB.get(i)));
-                writer.addEvent(a);
+                writer.addEvent(event);
             }
             ret += compare(banksA.get(i), banksB.get(i));
         }