RawDiffChunkGenerator now accepts either strings or lists of lines for

input, instead of requiring strings. When using lists of lines, we trust

that the content is already properly normalized and rendered to whatever

representative state may be wanted by the caller.
The caller can further customize behavior by changing normalization and

by overriding the logic to determine changes within a line.
This will make it easier for us to have specializations on chunk

generation behavior for specific cases.

Unit tests pass.
Used some of these new capabilities in an upcoming change.