checkheads: extract branchmap preprocessing The checkheads function is far too complicated. This extract help to explicite what part of the preprocessing are reused by the actual check. This the first step toward a wider refactoring.

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.

obsolete: add an any successors function This function yield every nodes which succeed to a group of nodes. The first user will be checkheads who need to know if we push successors for remote extra heads.

checkheads: extract bookmark computation from the branch loop No branch specific data are used in the computation of the bookmarked heads. We can only compute it once.