Make retrieving parents for mercurial commits work consistently across platforms

Summary:
Currently the template is single quoted, but Windows only supports double quotes. This meant that the output would be like:
  lang=text
  'aabbccddeeffaabbccddeeffaabbccddeeff0123
  '

Which is clearly wrong.

This is displayed like that in Phabricator as well, which is confusing.

Test Plan: ran `arc diff` on a Windows machine and saw the correct behaviour.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9450