diff: do not translate word-range indices by collect_unchanged_ranges()

Intersection of unchanged ranges becomes a simple merge-join loop, so I've
removed the existing tests. I also added a fast path for the common 2-way
diffs in which we don't have to build vec![(pos, vec![pos])].

One source of confusion introduced by this change is that WordPosition means
both global and local indices. This is covered by the added tests, but I'll
probably add separate local/global types later.