checkheads: take future obsoleted heads into account If we push some successors they will likely create a new head on remote. However as the obsoleted head will disappear after the push we are not really increasing the number of heads. There is several case which will lead to extra being actually pushed. But this first changeset aims to be simple. See the inline comment for details. Without this change, you need to push --force every time you want to push a newer version which is very error prone. The remote side still display +n heads on unbundle because it does not have the obsolete marker at unbundle time.