There's no need for an extra playbook for branch switchover

Change-Id: If4c88079960252c3ad0bb4c234965523d853aa93
diff --git a/playbooks/coverage-diff/pre.yaml b/playbooks/coverage-diff/pre.yaml
index e32243a..b9cc154 100644
--- a/playbooks/coverage-diff/pre.yaml
+++ b/playbooks/coverage-diff/pre.yaml
@@ -1,3 +1,4 @@
 - hosts: all
   roles:
     - ensure-pycobertura
+    - git-use-previous-commit