Improvement for the diff algorithm

As explained in [the corresponding article](https://fornever.me/en/posts/2021-02-12.constrained-diff-algorithm.html), we currently have a problem in the diff algorithm implementation: it won't step over several empty columns in one step, which leads to some transformations not being as efficient as they could (I believe there're a couple of examples in the current tests even).

This should be fixed.