Code review comment for lp:~mbp/bzr/415508-content-filtering

I think this patch is good.

I don't see a test added that would catch the bug that we discovered. Namely, that we *don't* add a new node for texts that have a content filter, but which have not otherwise changed.

I would like to see something along those lines, but I'm not going to block this patch on it, because I think the bug we have right now is pretty huge and needs fixing.