Add de-duplication to periodic triggers

Don't enqueue more than one entry for a project in a pipeline
that is triggered by a timer.  Match on the project name as that
is the only distinguishing attribute for a NullChange at the moment
(adding branch could be a future enhancement if we add multi-branch
support to periodic triggers).

Change-Id: I5b723497b23077652f2fc64b0c5367a20e3174c2
diff --git a/tests/fixtures/layout-idle.yaml b/tests/fixtures/layout-idle.yaml
new file mode 100644
index 0000000..e4574fa
--- /dev/null
+++ b/tests/fixtures/layout-idle.yaml
@@ -0,0 +1,12 @@
+pipelines:
+  - name: periodic
+    manager: IndependentPipelineManager
+    trigger:
+      timer:
+        - time: '* * * * * */1'
+
+projects:
+  - name: org/project
+    periodic:
+      - project-test1
+      - project-test2