compat: fix fixedcopytracing compatibility with mercurial 4.8 and narrow Mercurial 4.8 version of _fullcopytracing includes some new narrow code. As https://phab.mercurial-scm.org/D3896 has not yet landed, update fixedcopytracing behind a version detection condition. This was spotted by Augie Fackler.

# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)