Fix missing ChangeUtil.updated() call in PutTopic

The change was modified, it needs to update its timestamp.

Change-Id: I9024575cb589b29f2c275a36d5d444d75db631d3

gerrit-server/src/main/java/com/google/gerrit/server/change/PutTopic.java

@@ -103,6 +103,7 @@ class PutTopic implements RestModifyView<ChangeResource, Input>,
           @Override
           public Change update(Change change) {
             change.setTopic(Strings.emptyToNull(newTopicName));
+            ChangeUtil.updated(change);
             return change;
           }
         });