[SPARK-1955][GraphX]: VertexRDD can incorrectly assume index sharing

Fixes the issue whereby when VertexRDD's are `diff`ed, `innerJoin`ed, or `leftJoin`ed and have different partition sizes they fail under the `zipPartitions` method. This fix tests whether the partitions are equal or not and, if not, will repartition the other to match the partition size of the calling VertexRDD.

Author: Brennon York <brennon.york@capitalone.com>

Closes #4705 from brennonyork/SPARK-1955 and squashes the following commits:

0882590 [Brennon York] updated to properly handle differently-partitioned vertexRDDs

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

@@ -104,8 +104,14 @@ class VertexRDDImpl[VD] private[graphx] (
     this.mapVertexPartitions(_.map(f))
 
   override def diff(other: VertexRDD[VD]): VertexRDD[VD] = {
+    val otherPartition = other match {
+      case other: VertexRDD[_] if this.partitioner == other.partitioner =>
+        other.partitionsRDD
+      case _ =>
+        VertexRDD(other.partitionBy(this.partitioner.get)).partitionsRDD
+    }
     val newPartitionsRDD = partitionsRDD.zipPartitions(
-      other.partitionsRDD, preservesPartitioning = true
+      otherPartition, preservesPartitioning = true
     ) { (thisIter, otherIter) =>
       val thisPart = thisIter.next()
       val otherPart = otherIter.next()
@@ -133,7 +139,7 @@ class VertexRDDImpl[VD] private[graphx] (
     // Test if the other vertex is a VertexRDD to choose the optimal join strategy.
     // If the other set is a VertexRDD then we use the much more efficient leftZipJoin
     other match {
-      case other: VertexRDD[_] =>
+      case other: VertexRDD[_] if this.partitioner == other.partitioner =>
         leftZipJoin(other)(f)
       case _ =>
         this.withPartitionsRDD[VD3](
@@ -162,7 +168,7 @@ class VertexRDDImpl[VD] private[graphx] (
     // Test if the other vertex is a VertexRDD to choose the optimal join strategy.
     // If the other set is a VertexRDD then we use the much more efficient innerZipJoin
     other match {
-      case other: VertexRDD[_] =>
+      case other: VertexRDD[_] if this.partitioner == other.partitioner =>
         innerZipJoin(other)(f)
       case _ =>
         this.withPartitionsRDD(