branchmap: move validity logic in the object itself In several place, We check if a branchcache is still valid regarding the current state of the repository. This changeset puts this logic in a method of the object that can be reused when necessary. A branch map is considered valid whenever it is up to date or a strict subset of the repository state. The change will help making branchcache aware of filtered revision. The change in keyword is expected. the branch cache is actually invalid after the amend. The previous check did not detected it.