Change security notification code to use the new stream diff reporters

This ensures that even if security scanner pagination sends Old and New layer IDs on different pages, they will properly be handled across the entire notification.

Fixes https://www.pivotaltracker.com/story/show/136133657

test/test_morecollections.py

@@ -71,6 +71,17 @@ class IndexedStreamingDiffTrackerTests(unittest.TestCase):
 
     self.assertEquals(['a', 'c'], added)
 
+  def test_multiple_done(self):
+    added = []
+
+    tracker = IndexedStreamingDiffTracker(added.append, 3)
+    tracker.push_new([('a', 0), ('b', 1), ('c', 2)])
+    tracker.push_old([('b', 1)])
+    tracker.done()
+    tracker.done()
+
+    self.assertEquals(['a', 'c'], added)
+
   def test_same_streams(self):
     added = []
 
@@ -105,6 +116,20 @@ class IndexedStreamingDiffTrackerTests(unittest.TestCase):
 
     self.assertEquals(['a', 'b', 'c'], added)
 
+  def test_old_pagination_no_repeat(self):
+    added = []
+
+    tracker = IndexedStreamingDiffTracker(added.append, 2)
+    tracker.push_new([('new1', 3), ('new2', 4)])
+    tracker.push_old([('old1', 1), ('old2', 2)])
+
+    tracker.push_new([])
+    tracker.push_old([('new1', 3)])
+
+    tracker.done()
+
+    self.assertEquals(['new2'], added)
+
   def test_old_pagination(self):
     added = []