[libunwind] Set up .arcconfig to point to new Diffusion UNW repository

See http://lists.llvm.org/pipermail/cfe-dev/2017-November/056032.html
for related discussion and context.

Differential Revision: https://reviews.llvm.org/D41918

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@322228 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/.arcconfig b/.arcconfig
index 97a4bf1..78ee8d3 100644
--- a/.arcconfig
+++ b/.arcconfig
@@ -1,4 +1,4 @@
 {
-  "project_id" : "libunwind",
+  "repository.callsign" : "UNW",
   "conduit_uri" : "https://reviews.llvm.org/"
 }