Don't prompt for patches when outputting lint results as XML.

Summary: The Checkstyle XML output format is not intended to be an interactive workflow.

Test Plan: Introduced linter issues and ran `arc lint`. Verified that no interactive prompt was shown.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9107