Add method to DiffBuilder to allow appending from a DiffResult.

src/main/java/org/apache/commons/lang3/builder/DiffBuilder.java

@@ -949,6 +949,59 @@ public class DiffBuilder implements Builder<DiffResult> {
         return this;
     }
 
+    /**
+     * <p>
+     * Append diffs from another {@code DiffResult}.
+     * </p>
+     * 
+     * <p>
+     * This method is useful if you want to compare properties which are
+     * themselves Diffable and would like to know which specific part of
+     * it is different.
+     * </p>
+     * 
+     * <pre>
+     * public class Person implements Diffable&lt;Person&gt; {
+     *   String name;
+     *   Address address; // implements Diffable&lt;Address&gt;
+     *   
+     *   ...
+     *   
+     *   public DiffResult diff(Person obj) {
+     *     return new DiffBuilder(this, obj, ToStringStyle.SHORT_PREFIX_STYLE)
+     *       .append("name", this.name, obj.name)
+     *       .append("address", this.address.diff(obj.address))
+     *       .build();
+     *   }
+     * }
+     * </pre>
+     * 
+     * @param fieldName
+     *            the field name
+     * @param diffResult
+     *            the {@code DiffResult} to append
+     * @return this
+     */
+    public DiffBuilder append(final String fieldName,
+            final DiffResult diffResult) {
+        if (fieldName == null) {
+            throw new IllegalArgumentException("Field name cannot be null");
+        }
+        if (diffResult == null) {
+            throw new IllegalArgumentException("Diff result cannot be null");
+        }
+        if (objectsTriviallyEqual) {
+            return this;
+        }
+
+        for (Diff<?> diff : diffResult.getDiffs()) {
+            append(fieldName + "." + diff.getFieldName(),
+                   diff.getLeft(), diff.getRight());
+        }
+
+        return this;
+    }
+
     /**
      * <p>
      * Builds a {@link DiffResult} based on the differences appended to this

src/test/java/org/apache/commons/lang3/builder/DiffBuilderTest.java

@@ -411,6 +411,18 @@ public class DiffBuilderTest {
         assertEquals(0, list.getNumberOfDiffs());
     }
     
+    @Test
+    public void testDiffResult() {
+        final TypeTestClass class1 = new TypeTestClass();
+        final TypeTestClass class2 = new TypeTestClass();
+        class2.intField = 2;
+
+        final DiffResult list = new DiffBuilder(class1, class2, SHORT_STYLE)
+            .append("prop1", class1.diff(class2))
+            .build();
+        assertEquals(1, list.getNumberOfDiffs());
+        assertEquals("prop1.int", list.getDiffs().get(0).getFieldName());
+    }
 
     @Test(expected=IllegalArgumentException.class)
     public void testNullLhs() {