Refactor the object differ for use in tests. #2643.

This enables reliable tests that serialization / deserialization perfectly restores a Graph.
In some ways this simplifies the differ, but it also ensures that it performs a very deep recursive comparison of almost every object.
Also added tests that can compare two references to the same object, which provide strong evidence that the differ actually works.

src/main/java/org/opentripplanner/common/diff/Difference.java

@@ -17,39 +17,18 @@
 
 public class Difference {
 
-    public DiffType diffType;
-    public String property;
-    public Object oldValue;
-    public Object newValue;
+    Object a;
+    Object b;
+    String message;
 
-
-    public Difference(String property, Object oldValue, Object newValue) {
-        this.diffType = DiffType.VALUE_CHANGE;
-        this.property = property;
-        this.oldValue = oldValue;
-        this.newValue = newValue;
-    }
-
-    public Difference(DiffType diffType, String property, Object oldValue, Object newValue) {
-        this.diffType = diffType;
-        this.property = property;
-        this.oldValue = oldValue;
-        this.newValue = newValue;
+    public Difference(Object a, Object b) {
+        this.a = a;
+        this.b = b;
     }
 
-
-    public String toString() {
-
-        if (DiffType.COLLECTION_ADD.equals(diffType)) {
-            return property + ": added " + newValue;
-        }
-
-        if (DiffType.COLLECTION_REMOVE.equals(diffType)) {
-            return property + ": removed " + oldValue;
-        }
-
-
-        return property + ": " + oldValue + " => " + newValue;
+    public Difference withMessage (String formatString, Object... args) {
+        message = String.format(formatString, args);
+        return this;
     }
 
 }