Timeout interpreter {{{ ... }}} blocks after 15 seconds

Summary: Fixes T4149. This could be a little cleaner (configurable time limits, explicit timeout errors) but stop the major case of looping/infinite commands.

Test Plan: Added sleep 5 && and set timeout to 1, saw an error + kill.

Reviewers: btrahan, skyronic

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4149

Differential Revision: https://secure.phabricator.com/D7651