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.

mercurial/localrepo.py

@@ -263,6 +263,12 b' class localrepository(object):'
     def peer(self):
         return localpeer(self) # not cached to avoid reference cycle
 
+    def unfiltered(self):
+        """Return unfiltered version of the repository
+
+        Intended to be ovewritten by filtered repo."""
+        return self
+
     @filecache('bookmarks')
     def _bookmarks(self):
         return bookmarks.bmstore(self)