MyersDiffer had a bug where, if given an empty set of strings to

compare, it would yield an equal chunk. This could happen with

interdiffs when dealing with two deleted files, since both would end up

with empty strings.
The assumption was that if we reached the end of the processing and

haven't yielded anything, that it's an equal file. We'd then yield an

equal chunk with the ranges. In this case, the ranges would be 0,

causing index errors and bad assumptions later on. We now check for this

early and just bail.

Tested with some deleted files, which previously resulted in a single empty

chunk, causing an IndexError. After this, there were no chunks.