Dequeue changes when new patchset created.

When a new patchset is created for a change that is in a pipeline,
cancel running builds and dequeue that change (and possibly
dependent changes that can no longer merge).

Make this optional (and document the option).

Fixes bug 1022643.

Change-Id: I8f591956cf86645443e4b6075b8cdfc95a939e4f
Reviewed-on: https://review.openstack.org/20948
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
diff --git a/tests/fixtures/layout.yaml b/tests/fixtures/layout.yaml
index 586e2a5..cab97b9 100644
--- a/tests/fixtures/layout.yaml
+++ b/tests/fixtures/layout.yaml
@@ -31,6 +31,7 @@
 
   - name: unused
     manager: IndependentPipelineManager
+    dequeue-on-new-patchset: false
     trigger:
       - event: comment-added
         approval: