Add option to ignore ref-updated events emitted by branch deletions

When a branch is deleted, gerrit emits a ref-updated event with a newrev
value of all zeros. This adds a boolean field to optionally not trigger
on these ref-updated events.

Change-Id: Iea75d05ddcb49b0bf748b72b9d2d5472d077f0c6
diff --git a/tests/fixtures/layouts/good_layout.yaml b/tests/fixtures/layouts/good_layout.yaml
index fc2effd..9ba1806 100644
--- a/tests/fixtures/layouts/good_layout.yaml
+++ b/tests/fixtures/layouts/good_layout.yaml
@@ -20,6 +20,7 @@
       gerrit:
         - event: ref-updated
           ref: ^(?!refs/).*$
+          ignore-deletes: True
 
   - name: gate
     manager: DependentPipelineManager