Fix Diffusion rendering of SVN files which did not change

Summary:
Share code with the new PhabricatorDifferenceEngine, which handles diffs with no
changes correctly.

(This isn't the same issue as file moves, but I ran into it while generating a
repro case.)

Test Plan: Previously, changes which didn't change file content (e.g., property
changes) would throw. Now they work.
Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen, jungejason, aran
CC: aran, epriestley, tuomaspelkonen
Differential Revision: 698