Update monorepo .arcconfig with new project callsign.

llvm-svn: 352757
diff --git a/.arcconfig b/.arcconfig
index 92f8f45..48a85cd 100644
--- a/.arcconfig
+++ b/.arcconfig
@@ -1,3 +1,4 @@
 {
+  "repository.callsign" : "G",
   "conduit_uri" : "https://reviews.llvm.org/"
 }