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