Log which repo zuul references are created on

Change-Id: I2e0403ca52fded5ffe705cfa16358d3fa7141f0d
diff --git a/zuul/merger/merger.py b/zuul/merger/merger.py
index c84d042..1e881bf 100644
--- a/zuul/merger/merger.py
+++ b/zuul/merger/merger.py
@@ -168,7 +168,7 @@
 
     def createZuulRef(self, ref, commit='HEAD'):
         repo = self.createRepoObject()
-        self.log.debug("CreateZuulRef %s at %s" % (ref, commit))
+        self.log.debug("CreateZuulRef %s at %s on %s" % (ref, commit, repo))
         ref = ZuulReference.create(repo, ref, commit)
         return ref.commit