Bug 1460372 - Update .arcconfig to make history mutable. r=Fallen

Reviewers: Fallen

Reviewed By: Fallen

Bug #: 1460372

Differential Revision: https://phabricator.services.mozilla.com/D1252

--HG--
extra : rebase_source : 84686ff2a03e1dd50a68a5c1b23e794b9f13035e
extra : amend_source : b29f8835c433495534600e28e28f07b3a636a944

.arcconfig

@@ -1,4 +1,5 @@
 {
   "phabricator.uri" : "https://phabricator.services.mozilla.com/",
-  "repository.callsign": "COMMCENTRAL"
+  "repository.callsign": "COMMCENTRAL",
+  "history.immutable": false
 }