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

.arcconfig

@@ -1,5 +1,5 @@
 {
-	"repository.callsign" : "S",
+	"repository.callsign" : "G",
 	"phabricator.uri" : "https://reviews.freebsd.org/",
 	"arc.land.onto.default": "main",
 	"arc.land.onto": ["main"]