Set scriptandregex.script for reals when setting up an .arcrc

Summary:
(This is updated from the arcanist repo using the 'fake-subrepo' plan.)

We can assume that all new users will set up arcanist in
khan/devtools, because that's what the setup instructions say and
(very soon) what the setup script will do.  This is just one more step
users won't need to do by hand.

Test Plan: Fingers crossed

Reviewers: chris, benkomalo

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D2197