Fix PhutilInteractiveEditor if the user's $EDITOR contains space

Summary:
On linux if $EDITOR contains spaces, the editor will fail to launch. Use
%C instead of %s.

Test Plan:
arc diff works again with export EDITOR="/usr/bin/emacs -q"

Reviewers: epriestley, nh

Reviewed By: nh

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2055