Arcanist bash autocomplete

Summary:
Enable autocomplete for Arcanist to use with arc shell.

Fixes T633.

Test Plan: Run the contain, then type arc bra followed by a <tab>.

Reviewers: dereckson

Subscribers: Nasqueron Docker deployment squad

Projects: Docker images

Maniphest Tasks: T633

Differential Revision: http://devcentral.nasqueron.org/D131