Set "history.immutable" to "false" explicitly in .arcconfig in Arcanist

Summary: Depends on D16364. See that revision for discussion.

Test Plan: Ran `arc get-config history.immutable` in `arcanist/`, saw setting.

Reviewers: chad, fooishbar

Reviewed By: fooishbar

Differential Revision: https://secure.phabricator.com/D16365

.arcconfig

@@ -2,5 +2,6 @@
   "phabricator.uri": "https://secure.phabricator.com/",
   "load": [
     "src/"
-  ]
+  ],
+  "history.immutable": false
 }