clfilter: introduce an `unfilteredmethod` decorator
This decorator ensure the method in run on an unfiltered version of the
repository. See follow-up commit for details.
This decorator is not named `unfiltered` because it would clash with the
`unfilteredmethod` on `localrepo` itself.
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])