Improve handling for renamed files

Currently when I click on a png file that's listed as R (which is apparently a Renamed file), I get:

Ideally:

• I should be able to view the file locally and have a thing that tells me its renamed at the top/bottom of the view
• I should be able to add comments on the file change (as I can in the GitHub web ui)

In the interim, it'd be nice if the floating message at the bottom said "because it's a rename" -- so it's clear it's because of the R and not, e.g. because it's a .png.