Bug 1424950: Add missing repository.callsign to .arcconfig. r=dkl

Summary: MozReview-Commit-ID: 74DSn04pBig

Reviewers: dkl

Bug #: 1424950

Differential Revision: https://phabricator.services.mozilla.com/D335

MozReview-Commit-ID: 3hdaZ2s1MjL

.arcconfig

@@ -1,3 +1,4 @@
 {
-  "phabricator.uri" : "https://phabricator.services.mozilla.com/"
+  "phabricator.uri" : "https://phabricator.services.mozilla.com/",
+  "repository.callsign": "MOZILLACENTRAL"
 }