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