Fix arc land for mercurial on windows

Summary:
arc for mercurial on windows was broken in several way.
Executing a command via passthru failed because passthru on windows
skips the shell so 'set HGPLAIN=1 & ...' was an invalid command. The
fix was to just not set HGPLAIN for passthru commands on windows.

Also removed hardcoded '' quotes in mercurial commands since windows
doesn't support single quots.

Test Plan: arc land --hold on a windows machine

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6763