Don't set ahead/behind on independent queues.

Change-Id: I6507896e9980eee4ac13ff8bd94f0e32947cd5d8
Reviewed-on: https://review.openstack.org/12279
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
diff --git a/zuul/model.py b/zuul/model.py
index f9314d7..7deef06 100644
--- a/zuul/model.py
+++ b/zuul/model.py
@@ -255,7 +255,7 @@
             self._jobs |= set(self.pipeline.getJobTree(project).getJobs())
 
     def enqueueChange(self, change):
-        if self.queue:
+        if self.dependent and self.queue:
             change.change_ahead = self.queue[-1]
             change.change_ahead.change_behind = change
         self.queue.append(change)