mergestate: make clean() only be about creating a clean mergestate
This is similar to the previous patch, but moves the separation one
step further out, to `merge.py`.
Differential Revision: https://phab.mercurial-scm.org/D9034
# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)