git-hgext: adjust to the lack of `changelog.heads` method
We don't have a `heads` method returning nodeid, but this is very easy to get
the same result.
This was flagged by pytype.
We can note that the fact this code did not break is probably a good sign that
it is dead code.
However this is a question outside of the scop of this series.
# __init__.py - Startup and module loading logic for Mercurial.
#
# Copyright 2015 Gregory Szorc <gregory.szorc@gmail.com>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.
# Allow 'from mercurial import demandimport' to keep working.
import hgdemandimport
demandimport = hgdemandimport