Restore the old "history.immutable" default behavior

Summary: See rARC104219dd. Per the explanatory text, this should default to `true` in Mercurial and `false` in Git.

Test Plan: Ran `arc get-config` in Mercurial repo.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10021
Patch: don't complain about reviewers not being specified.
This will break in the future when upstream arc incorporates T4631. Hopefully there will be a better solution by then