Restore the old "history.immutable" default behavior
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

codelabs-ch/arcanist,wxstars/arcanist,uber/arcanist,wxstars/arcanist,aik099/arcanist,aik099/arcanist,dguo-coursera/arcanist,KieranHayes/arcanist,sneakpeeq/arcanist,memsql/arcanist,dguo-coursera/arcanist,aik099/arcanist,kevinseelbach/arcanist,chundongwang/arcanist,alexmv/arcanist,jra3/arcanist,phacility/arcanist,KieranHayes/arcanist,wikimedia/phabricator-arcanist,alexmv/arcanist,sneakpeeq/arcanist,haskell-infra/arcanist,haskell-infra/arcanist,r4nt/arcanist,zhihu/arcanist,memsql/arcanist,phacility/arcanist,coursera/arcanist,r4nt/arcanist,kevinseelbach/arcanist,r4nt/arcanist,uber/arcanist,wxstars/arcanist,PageUpPeopleOrg/arcanist-patches,phacility/arcanist,calblueprint/arcanist,wxstars/arcanist,eSpark/arcanist,alexmv/arcanist,wharris/arcanist,wikimedia/phabricator-arcanist,coursera/arcanist,jra3/arcanist,folsom-labs/arcanist,PageUpPeopleOrg/arcanist-patches,zhihu/arcanist,dguo-coursera/arcanist,eSpark/arcanist,Khan/arcanist,alexmv/arcanist,dguo-coursera/arcanist,Symplicity/arcanist,folsom-labs/arcanist,wikimedia/phabricator-arcanist,fivestars/arcanist,sneakpeeq/arcanist,zhihu/arcanist,r4nt/arcanist,wikimedia/phabricator-arcanist,kevinseelbach/arcanist,KieranHayes/arcanist,phacility/arcanist,calblueprint/arcanist,Khan/arcanist,memsql/arcanist,aik099/arcanist,uber/arcanist,fivestars/arcanist,uber/arcanist,chundongwang/arcanist,haskell-infra/arcanist,wharris/arcanist,sneakpeeq/arcanist,jra3/arcanist,fivestars/arcanist,Symplicity/arcanist,hach-que/arcanist,calblueprint/arcanist,PageUpPeopleOrg/arcanist-patches,chundongwang/arcanist,codelabs-ch/arcanist,wharris/arcanist,Khan/arcanist,Symplicity/arcanist,Symplicity/arcanist,zhihu/arcanist,wharris/arcanist,haskell-infra/arcanist,eSpark/arcanist,hach-que/arcanist,folsom-labs/arcanist,PageUpPeopleOrg/arcanist-patches,Khan/arcanist,calblueprint/arcanist,codelabs-ch/arcanist,chundongwang/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,phacility/arcanist,fivestars/arcanist,coursera/arcanist,eSpark/arcanist,memsql/arcanist,coursera/arcanist,folsom-labs/arcanist,jra3/arcanist,hach-que/arcanist,hach-que/arcanist,kevinseelbach/arcanist,KieranHayes/arcanist