filemerge: rename backup variables from `back` to `backup`
`backup` seems like an obviously clearer name for something containing
something related to a backup.
Differential Revision: https://phab.mercurial-scm.org/D11876
# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)