branchmap: move the cache file name into a dedicated function Filtered view of the repo will want to write they file name in a different file.

branchmap: read and write key part related to filtered revision Now that we have a third part for the cache key we need to write and read it on disk. It is only written when there is filtered revision. This keep the format compatible with older version. Notes that, at this state, filtered repository does not use any disk caches yet.

check-code: disallow defunct <> operator Added a test for that and one other python3 check in test-check-code.t.