Fix hg 'arc land' when the mq extension is not enabled

Summary:
Mercurial 'arc land' uses the 'hg strip' command to clean up after
itself, but this command isn't available unless the mq extension is enabled.
The fix is to enable it for that particular command only.

Test Plan: Ran 'arc land' with the mq extension disabled. It worked.

Reviewers: epriestley, bos, sid0, dschleimer

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4955