arc: Make the history immutable

Under an immutable history the history will not be rewritten.

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

.arcconfig

@@ -1,3 +1,4 @@
 {
   "phabricator.uri" : "https://bugs.libssh.org/"
+  "history.immutable": true
 }