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