Allow Filesystem::walkToRoot to walk to a given root directory
Summary: Allow Filesystem::walkToRoot to walk to a given root directory. See D13185#139299.
Test Plan: Added unit tests.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D13202