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

repoview: add a subset table This will be used by branchmap collaboration. See inline documentation for more details

branchmap: add a copy method If we want branchcache of different filter to collaborate, they need a simple way to copy each other. This will ensure that each filtered have no side effect on other filter level cache.

clfilter: use empty frozenset intead of empty tuple This will allows set operation needed for cache collaboration.