Introduce "RepositoryLocalState", a modern version of "requireCleanWorkingCopy()"

Summary:
Ref T13546. Introduces a more structured construct for saving and restoring local repository state.

This is similar to old behavior, except that:

• "arc.autostash" is no longer respected;
• untracked changes are stashed; and
• we do not offer to amend.

Test Plan: In future changes, saved and restored various permutations of local state.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21314