branchmap: enable caching for filtered version too The `_branchcache` attribute is turned into a dictionary. Key are filter name and value is a `branchcache` object. Unfiltered version is cached as `None` filter. The attribute is renamed to `_branchcaches` to avoid confusion with the previous one. Both old and new contents are dictionary even if their contents are different. I prefer possible extension code to crash right away instead of just messing the wrong dictionary. As all different caches work isolated to each other, this code keeps the previous behavior of using the unfiltered cache we nothing is filtered. This is a cheap way to have cache collaborate and nullify potential impact in the default case.

branchmap: report filtername when read fails Now that we can have multiple one, we need to know which filecache failed to be read from disk.

branchmap: use a different file name for filtered view of repo

clfilter: ensure unfiltered repo have a filtername attribute too That will allows to use `repo.filtername` for dispatch purpose.

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.