Enable lint amending commits in mercurial

Summary:
arc lint was hardcoded for git for amending commits with lint
patches. This enables the same functionality for mercurial.

Test Plan:
Made some changes that would result in a lint patch.
arc diff

Verify that the patches it produces were amended into the commit.

Verified it still works in git as well.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5716