Call clearstatcache() after changing ~/.arcrc permissions

Summary: Otherwise we'll get a cached result from fileperms() if we end up here again.

Test Plan: chmod 644 ~/.arcrc ; arc help no longer double prompts when run from outside of a .arcconfig working copy.

Reviewers: csilvers

Reviewed By: csilvers

CC: aran

Maniphest Tasks: T1359

Differential Revision: https://secure.phabricator.com/D2752