corrected method by leveraging bitmask operations to correctly return only the portions of that are different from the calling VertexRDD

Author: Brennon York

graphx/src/main/scala/org/apache/spark/graphx/impl/VertexPartitionBaseOps.scala

@@ -93,32 +93,8 @@ private[graphx] abstract class VertexPartitionBaseOps
    * the values from `other`.
    */
   def diff(other: Self[VD]): Self[VD] = {
-//    if (self.index != other.index) {
-//      logWarning("Diffing two VertexPartitions with different indexes is slow.")
-//      diff(createUsingIndex(other.iterator))
-//    } else {
-//      val newMask = self.mask & other.mask
-/*      logInfo("NEWMASK START " + other.iterator.mkString + " AND " + 
-              self.iterator.mkString)
-      var i = newMask.nextSetBit(0)
-      while (i >= 0) {
-        if (self.values(i) == other.values(i)) {
-          logInfo("BEFORE i: " + i + " newMask: " + newMask.iterator.mkString + 
-                  " sv " + self.values(i) + " ov " + other.values(i) + " om: " +
-                other.mask.iterator.mkString + " selfm: " + 
-                self.mask.iterator.mkString)
-          newMask.unset(i)
-        }
-        i = newMask.nextSetBit(i + 1)
-      }*/
-    /*
-      this
-        .withIndex(other.index)
-        .withValues(other.values)
-        .withMask((self.mask & other.mask) ^ other.mask)
-        * */
+    // SPARK-4600: Use bitmask operations to only show the values in `other`.
     other.withMask((self.mask & other.mask) ^ other.mask)
-//    }
   }
 
   /** Left outer join another VertexPartition. */