Fix "arc" when `arcanist/` lives in some directory with spaces in the name

Summary: This shell script needs more quotes.

Test Plan: Ran "arc" with `arcanist/` inside a directory called `s p a c e s`.

Reviewers: irinav, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D5265