Fix a breakage with interdiffs.
The new interdiff code had a few occasions where it'd miss chunks. It

didn't actually miss them, it just miscategorized as equals.
The reason it did this was that it was attempting to figure out the next

range to consider after figuring out if a chunk was valid, and was only

considering the very next range. What it didn't consider was that there

may be a handful of ranges it should be skipping over that are equal

between the interdiffs, and that it may need to figure this out before

determining if a chunk is valid
This fixes the broken interdiff noticed on

https://reviews.reviewboard.org/r/4257/.
A unit test was added for this case.

All unit tests pass.
Checked my sample broke interdiffs. They appeared to work correctly.