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