Summary:
Adds data-driven shell completion help to arcanist.
Test Plan:
ran various commands in git and svn working copies,
output seemed reasonable
Differential Revision: 201754
Reviewed By: adonohue
Reviewers: mroch, adonohue
Commenters: crackerjack
CC: epriestley, adonohue, achao
Revert Plan:
OK
