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.
$ "$TESTDIR/hghave" pyflakes || exit 80
$ cd "`dirname "$TESTDIR"`"
$ pyflakes mercurial hgext 2>&1 | "$TESTDIR/filterpyflakes.py"
hgext/inotify/linux/__init__.py:*: 'from _inotify import *' used; unable to detect undefined names (glob)