Improve Diffusion behavior for SVN file moves

Summary:
We just weren't handling these at all reasonably, must have dropped the logic
when they got ported.

This still isn't perfect: we have some display glitches around file names, so
the 'away' part renders as "This file was moved to .". I'll see if I can follow
up and fix that, but this resolves the more immediate issue of the interface
just not working at all.

Test Plan: Moved and copied files in my test repository, verified they rendered
somewhat correctly.
Reviewed By: jungejason
Reviewers: tuomaspelkonen, jungejason, aran
CC: aran, jungejason, epriestley
Differential Revision: 699