bugfix: overflow of graphx Edge compare function

Author: Zhen Peng <zhenpeng01@baidu.com>

Closes #769 from zhpengg/bugfix-graphx-edge-compare and squashes the following commits:

8a978ff [Zhen Peng] add ut for graphx Edge.lexicographicOrdering.compare
413c258 [Zhen Peng] there maybe a overflow for two Long's substraction

graphx/src/main/scala/org/apache/spark/graphx/Edge.scala

@@ -56,7 +56,13 @@ case class Edge[@specialized(Char, Int, Boolean, Byte, Long, Float, Double) ED]
 
 object Edge {
   private[graphx] def lexicographicOrdering[ED] = new Ordering[Edge[ED]] {
-    override def compare(a: Edge[ED], b: Edge[ED]): Int =
-      (if (a.srcId != b.srcId) a.srcId - b.srcId else a.dstId - b.dstId).toInt
+    override def compare(a: Edge[ED], b: Edge[ED]): Int = {
+      if (a.srcId == b.srcId) {
+        if (a.dstId == b.dstId) 0
+        else if (a.dstId < b.dstId) -1
+        else 1
+      } else if (a.srcId < b.srcId) -1
+      else 1
+    }
   }
 }

graphx/src/test/scala/org/apache/spark/graphx/EdgeSuite.scala

+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.graphx
+
+import org.scalatest.FunSuite
+
+class EdgeSuite extends FunSuite {
+  test ("compare") {
+    // decending order
+    val testEdges: Array[Edge[Int]] = Array(
+      Edge(0x7FEDCBA987654321L, -0x7FEDCBA987654321L, 1), 
+      Edge(0x2345L, 0x1234L, 1), 
+      Edge(0x1234L, 0x5678L, 1), 
+      Edge(0x1234L, 0x2345L, 1), 
+      Edge(-0x7FEDCBA987654321L, 0x7FEDCBA987654321L, 1)
+    )
+    // to ascending order
+    val sortedEdges = testEdges.sorted(Edge.lexicographicOrdering[Int])
+    
+    for (i <- 0 until testEdges.length) {
+      assert(sortedEdges(i) == testEdges(testEdges.length - i - 1))
+    }
+  }
+}