Short-circuit diff patching for moved files with no changes.

The comment for `DiffChunkGenerator.get_chunks()` indicates that moved files
with no changes should be returned as an empty list without actually fetching
source files or applying patches, but this wasn't the case. This change fixes
the conditional to return the empty list.

Testing done:
- Tested a moved file with no changes. Saw that it displayed correctly without
  ever calling out to 'patch'
- Tested a moved+edited file. Saw that it displayed correctly.
- Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/6655/