Default the prompt for "Amend HEAD with these patches" to true

Summary:
Most users, if they have gone through the trouble of
accepting the auto-fixes, are most likely going to want to take those
changes and attempt to land with them. Assuming "Y" for this prompt
streamlines for the more likely flow.

Test Plan: arc lint

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D18824