Merge changes Id255aa3d,I37f156cd

* changes:
  Introduce server.util.DelegateSystemReader
  AbstractChangeNotes: mark readRef @Nullable

java/com/google/gerrit/server/notedb/AbstractChangeNotes.java

@@ -153,6 +153,7 @@ public abstract class AbstractChangeNotes<T> {
     return self();
   }
 
+  @Nullable
   protected ObjectId readRef(Repository repo) throws IOException {
     Ref ref = repo.getRefDatabase().exactRef(getRefName());
     return ref != null ? ref.getObjectId() : null;