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