Arcanist - allow amending revisions by other authors in the working copy

Summary: Fixes T4670. Give the user an option to abort but otherwise proceed.

Test Plan: arc patch D8685 (by epriestley); arc amend

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4670

Differential Revision: https://secure.phabricator.com/D8716