Provide a getProjectRoot method for ArcanistLinter

Summary: Adds a getProjectRoot() method to the ArcanistLinter class, simplifying a bunch of code.

Test Plan: arc unit

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12474