arcconfig: add callsign again

Problem

When using git-svn or other non-pure-svn tooling the original subversion
URL is not present. This causes arcanist/phabricator to be unable to
determine which repository is being modified.

Solution

Restore callsign to .arcconfig to enable exact repository matching even
with git-svn.

Reported By:	jhb

.arcconfig

@@ -1,4 +1,5 @@
 {
+	"repository.callsign" : "S",
 	"phabricator.uri" : "https://reviews.freebsd.org/",
 	"history.immutable" : true
 }