Fix problem with duplicate change suppression.

Jobs for changes weren't being launched for DependentQueueManagers
because its addChange method called the superclass's addChange,
which had the same duplicate suppresion logic.  This breaks
out the bulk of that into a private method that can be called
by both classes.

Change-Id: Ib47be82f255bab38ce03c7af7fb5b700e1956aaa
diff --git a/zuul/scheduler.py b/zuul/scheduler.py
index 7b28dec..2f8c491 100644
--- a/zuul/scheduler.py
+++ b/zuul/scheduler.py
@@ -415,6 +415,9 @@
             self.log.debug("Change %s is already in queue, ignoring" % change)
             return
         self.log.debug("Adding change %s" % change)
+        self._addChange(change)
+
+    def _addChange(self, change):
         if self.start_action:
             try:
                 self.log.info("Reporting start, action %s change %s" % (
@@ -600,7 +603,7 @@
             self.log.debug("Adding change %s to queue %s" % (
                     change, change_queue))
             change_queue.enqueueChange(change)
-            super(DependentQueueManager, self).addChange(change)
+            self._addChange(change)
 
     def _getDependentChanges(self, change):
         orig_change = change