Don't reverse downloaded raw diff

Summary:
The logic here was swapped - new file should be on the right side.
Plus we had a fatal for VS = -1 where new file should be on left.

Test Plan:
Downloaded raw diff of:

• base VS change
• change VS change
• change VS change with unmodified file

Reviewers: epriestley, btrahan

Reviewed By: btrahan

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3333