Comment 3 for bug 82233

Well, we have a switch to prevent changing the mainline (something like append_only=True).

I would probably go for a simple config item. I believe you could do it in a plugin, especially now that we have a hook for whenever the revision_id of a branch changes. (Unless that cannot prevent the change, and only reports that it *does* change.) We also have a push and pull hook that could probably handle some of it (you also need a commit hook, etc)