Clean up a PhutilURI "alter()" callsite in Diffusion blame
Summary: See https://discourse.phabricator-community.org/t/exception-when-viewing-previous-revision-from-blame/2454/2.
Test Plan: Viewed blame, clicked "Skip Past This Commit". Got jumped to the right place instead of a URI exception.
Reviewers: amckinley
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D20222