topicmap: move the monkey patching into a context manager There is a couple of other place doing branchmap/topicmap update (of the served set), we'll have to set the monkey patching for them. This changeset is just doing the move to a context manager to make sure it is correct.

push: hackish handeling of new branch head from phase move The current head checking mechanism is not expecting "head change" from phase movement. Topic allows that, changeset with a topic moving to public can create a new head. We introduce a hack to double check that no head were added at the transaction level to work around this.