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