Set history.immutable to false in .arcconfig.

Summary:
Although this setting should affect the current test plan, we
generally make history mutable in our repos on Phabricator, and
it lets devs do more accurate testing with this repo.

Add test file.

Reviewers: dkl

Reviewed By: dkl

Differential Revision: https://phabricator-dev.allizom.org/D753