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.

obsolete: mark unreachable extinct changesets as hidden The repo.hiddenrevs set is updated with all extinct() changesets which aren't descendants of either: - the current working copy, - a bookmark, - a tag.

hidden: move hiddenrevs set on the repository This set is always accessed through the repo for now. Having this set carried by the changelog make it complicated to: - initialize it, computing hidden set may involve revset call - lazy compute it, (1) only the changelog can detect someone access it, (2) only the repo have enought knowledge to compute it. In later version I expect he changelog to apply filtering itself and the set to be carried by changelog again.