Correct line highlighting behavior in Diffusion

Summary: See https://discourse.phabricator-community.org/t/line-highlighting-in-diffusion-breaks-url/1207. Ref T13088. This was disrupted by changes for the new Harbormaster build logs and now needs an explicit base URI.

Test Plan: Clicked lines and dragged across line ranges in Diffusion, observed correct URI behavior.

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19187