It's possible to hit an IndexError when processing an interdiff between two

changes that delete a file. The wrinkle here is that between the two revisions

of the diff, some additional changes to that file were made (and synced to the

local developer's tree).
The result of this is that we didn't filter out the file from the list of

things to show for the interdiff, and then things went wrong when we tried to

process the opcodes for diffing two empty strings.
While I was doing this, I saw some warnings about a unicode-unaware comparison

occurring, so I've slipped the fix for that in as well. This doesn't actually

have any correctness implications, it just speeds up things slightly when

dealing with empty files.

Created a reproduction case for the bug and tested before and after the

  change. Before, I saw an entry for this file with the error inside. After,

  the deleted files don't show up at all in the interdiff.
Ran unit tests.