Refactor ChangeMessage to include PatchSet.Id

Requires a PatchSet.Id to be included when instantiating ChangeMessage.
PatchSet.Id can be null if necessary, for instance in the
case of MergeOp messages.

Change-Id: I2d6d980d986f72a5aed2de6443632a47e6238215

gerrit-server/src/main/java/com/google/gerrit/server/patch/PublishComments.java

@@ -265,7 +265,7 @@ public class PublishComments implements Callable<VoidResult> {
     msgbuf.append(messageText != null ? messageText : "");
 
     message = new ChangeMessage(new ChangeMessage.Key(change.getId(),//
-        ChangeUtil.messageUUID(db)), user.getAccountId());
+        ChangeUtil.messageUUID(db)), user.getAccountId(), patchSetId);
     message.setMessage(msgbuf.toString());
     db.changeMessages().insert(Collections.singleton(message));
   }