Update directory mtimes when a file inside changes

Summary: Reviewed over the shoulder by @wez.

Test Plan: `arc unit --everything`

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D871378

--HG--
extra : rebase_source : 2fd79ec883a0773fd20350a92f5976f20c46b959
extra : amend_source : 9cac0fc8abc401dc51cd8c346fb9ea66fb5cb36a