Fix is_absolute test in markup

Summary:
See Q59. Fixes rP935d7120ee32.

Call sites should be happy to use PhutilURI when possible.

Test Plan: visit any Repository page. Before - exception, now - data.

Reviewers: O1 Blessed Committers!, valerio.bozzolan

Reviewed By: valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25139