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.

Imported from:

https://secure.phabricator.com/rARC0fc22183e796fb8ac2e3a0a3f3f37aa964c6d7fa

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.

Closes T15288

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15288

Differential Revision: https://we.phorge.it/D25143