branchmap: allow to use cache of subset Filtered repository are *subset* of unfiltered repository. This means that a filtered branchmap could be use to compute the unfiltered version. And filtered version happen to be subset of each other: - "all() - unserved()" is a subset of "all() - hidden()" - "all() - hidden()" is a subset of "all()" This means that branchmap with "unfiltered" filter can be used as a base for "hidden" branchmap that itself could be used as a base for unfiltered branchmap. unserved < hidden < None This changeset implements this mechanism. If the on disk branchcache is not valid we use the branchcache of the nearest subset as base instead of computing it from scratch. Such fallback can be cascaded multiple time is necessary. Note that both "hidden" and "unserved" set are a bit volatile. We will add more stable filtering in next changesets. This changeset enables collaboration between no filtering and "unserved" filtering. Fixing performance regression introduced by 47f00b0de337