hgext/__init__.py

rust-cpython: binding for headrevs()

This uses the core `dagops::retain_heads` to give a Rust implementation
to `mercurial.dagop.headrevs`.

Testing happens for now from `test-rust-ancestors.py`
(for quick and minimal change), but it'd made more sense to put the binary
index data elsewhere and to create a new test python module

from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)