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.

clfilter: introduce an "unfiltered" method on localrepo This commit is part of the changelog level filtering effort. It returns the main "unfiltered" version of a repo-like object. For localrepo this means the same localrepo object. But this method will be overwritten by the filtered versions of a repository to return the core unfiltered version of the repo. Introducing this simple method first allows later commits to prepare for the use of a filtered version of a repository. A new repo method is added because a lot of users may call it. At the end of this series of commits, about 40 calls exist in core and hgext.