fix: deleted trailing newline causes panic

[tommilligan]

Closes #96

Turns out that we had assumed an invariant from the underlying diff library that does not, in fact, exist. We can receive an insertion followed by a deletion in the case where a trailing newline is deleted - for some reason the trailing newline deletion is not grouped with the previous deletion, and instead follows after the insertion on its own, despite there being no plain text in the middle.

It's a slightly odd looking diff, but I think it makes sense and is not worth special casing further. It's handled nicely by just removing our invariant check and letting the existing logic handle it.