Name extension as "arc-hg", not "arg-hg"

Summary: Ref T13659. I don't think this name actually matters since Mercurial doesn't care what the extension name is, but be a little more consistent.

Test Plan: Ran "arc land ..." and saw it run "arc-hg" commands. There's no behavioral change here: Mercurial loads the extension and runs fine no matter what name we provide.

Reviewers: cspeckmim

Reviewed By: cspeckmim

Maniphest Tasks: T13659

Differential Revision: https://secure.phabricator.com/D21711