arcanist: use FreeBSD/git project repository instead of FreeBSD/svn

Current .arcconfig specifies explicit mapping to the FreeBSD/SVN
project in Phabricator, which is inactive. This mapping only gets
updated to the current "production" (FreeBSD/git) project when the
underlying change is committed.
Update arcanist configuration to create all new diffs in the
FreeBSD/git project.

Reviewed By: dim, emaste
Differential Revision: https://reviews.freebsd.org/D36204