(stable) Update the arc-hg.py extension to work with mercurial 6.4

Summary:
Mercurial 6.4 was recently released and showing up in package managers. With
the update to 6.4 using arc land would result in an exception indicating that
expandpath function does not exist.

The ui.expandpath function was deprecated in 5.8 and now removed in 6.4. The
functionality has been moved to utils.urlutil.get_ functions (they are split
between getting pull, push, and clone paths).

This updates the script to try utils.urlutil.get_clone_path function if the
ui.expandpath function is not present.

Test Plan:
I updated my latest mercurial install to 6.4 and verified with hg --version.

I created a diff in a mercurial repo and used arc land to successfully land
the revision without any exceptions.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D21859