Align usages of "\1" and "\2" in MercurialRepositoryAPI

Summary:
Especially on Windows it is hard to use "\1" type escapes in shell commands. The direct usage resulted in some undefined variables because the \1 and \2 weren't actually passed as control characters.

By passing them through the regular arguments list they get sent in the "correct way" regardless of OS

Test Plan: Executed `arc diff` in a HG repo and did not get undefined indexes back

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8129