Arcanist - fix shouldAmend code

Summary: make it a real member variable. See rARC77a9c1814063 and D8753#33914.

Test Plan: sending up this diff!

Reviewers: shadowhand, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8761