Add coverage tooltips in Diffusion file browse mode

Summary: Fixes T10816. The way these work is a little unusual since these chunks of file-rendering code are unusuall performance-sensitive, so the Differential version doesn't adapt directly to Diffusion. Both can possibly be unified at some point in the future, although they do slightly different things.

Test Plan: {F1220170}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10816

Differential Revision: https://secure.phabricator.com/D15719