When processing an interdiff, we filter out code we don't want to

specially highlight as "filtered-equal" chunks. These used to be ignored

by indentation detection, but Review Board 4.0.6 gained the ability to

show indentation changes for these lines, with the expectation being

that we'd show them for "equal" lines marked as "filtered-equal".

Unfortunately, this broke the indentation detection, which could see two

non-equal lines as having an indentation change.
This led to a crash in the chunk generator, where indentation

serialization code would expect to be able to iterate over a prepared

string of indentation characters, and would then reference the last

index processed during that iteration. In this case, the string was

empty, causing the loop to never run, and this caused a variable that

was needed to not yet be defined.
A few things were done to address this issue:


The indentation calculations now compare strings after stripping

   them, ensuring that they still appear to be equal and safe for

   indentation calculation.


The chunk generator now checks to ensure that the resulting

   indentation ranges aren't 0 (which is what we'd see in the failing

   case). While this should not be an issue anymore post-4.0.7, we could

   be dealing with something in cache. In this case, we'll end up still

   showing the chunks as their own empty uncollapsed range of lines in

   the diff viewer, but it's better than crashing.


The indentation serialization code now asserts that the string is

   non-empty, so we don't hit the original crash case.

Reproduced this with a consistent failure case, and a verified the

fix (with and without the bad state in cache).
Unit tests pass.