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/layout-dont-ignore-deletes.yaml b/tests/fixtures/layout-dont-ignore-deletes.yaml
new file mode 100644
index 0000000..1cf3c71
--- /dev/null
+++ b/tests/fixtures/layout-dont-ignore-deletes.yaml
@@ -0,0 +1,16 @@
+includes:
+  - python-file: custom_functions.py
+
+pipelines:
+  - name: post
+    manager: IndependentPipelineManager
+    trigger:
+      gerrit:
+        - event: ref-updated
+          ref: ^(?!refs/).*$
+          ignore-deletes: False
+
+projects:
+  - name: org/project
+    post:
+      - project-post
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