Refactor getContextDiffNumbered with separate Nothing equation

[ninioArtillero]

Adding an extra equation for the Nothing case lifts the burden of handling it at doPreffix and doSuffix, resulting in clearer code and avoiding unnecessary traversals of getGroupedDiff a0 b0 output. The key insight is that the Nothing case results in all elements being preserved by doPrefix+doSuffix, i.e. no Both is split, and thus producing a single hunk with the output of getGroupedDiff.

It is also of note that calls to max were already no-ops as drop truncates negatives to 0.

[ninioArtillero]

@ddssff This is ready for your review 🙂

[ddssff]

Any thoughts about #1?