remotefilelog: use raw strings when looking for variable names
Keys in self.__dict__ and vars() are always str. So we need to use
raw strings to ensure lookups work on Python 3.
# skip-blame just r'' prefixes
Differential Revision: https://phab.mercurial-scm.org/D5957
Georges Racinet <georges.racinet@octobus.net> [Thu, 10 Jan 2019 18:25:18 +0100] rev 41705
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