Fully specify default branch name

Change-Id: Ib8aad0d2a6c9597aef6ad6e3e04df0cee8dd10ce
diff --git a/src/main/java/com/amd/gerrit/plugins/manifestsubscription/Utilities.java b/src/main/java/com/amd/gerrit/plugins/manifestsubscription/Utilities.java
index fb6608b..5c5ce00 100644
--- a/src/main/java/com/amd/gerrit/plugins/manifestsubscription/Utilities.java
+++ b/src/main/java/com/amd/gerrit/plugins/manifestsubscription/Utilities.java
@@ -148,7 +148,7 @@
     MetaDataUpdate update = metaDataUpdateFactory.create(p);
     if (commitId == null) {
       // TODO remove assumption that master branch always exists
-      vManifests = new VersionedManifests("master");
+      vManifests = new VersionedManifests("refs/heads/master");
       vManifests.load(update);
     } else {
       vManifests = new VersionedManifests(manifestBranch);