Configure Arcanist repository

Arcanist configuration files start by .arc and then their goal,
hence two c at .arcconfig.

References:

• https://devcentral.nasqueron.org/w/new-repo/
• https://secure.phabricator.

Summary:
...or.com/book/phabricator/article/arcanist/

This is a follow-up for bc7bacc3538a.

Test Plan: arc which (issue initially spot by ec to test EditorConfig)

Reviewers: inidal

Differential Revision: https://devcentral.nasqueron.org/D3178