Fix an intradiff error when the newer changeset does not exist
Summary: Ref T13523. If a file hasn't been touched in the newer changeset, we can currently hit an error in the interdiff.
Test Plan:
- Touched "moo.txt" in Diff 1.
- Reverted the changes to "moo.txt" in Diff 2.
- Diffed 2 vs 1.
- Before patch: fatal (call to getFilename() on null).
- After patch: clean interdiff.
Maniphest Tasks: T13523
Differential Revision: https://secure.phabricator.com/D21220