DiffChunkGenerator is a very useful and powerful class for producing a

set of chunks we can directly use for rendering diffs. To use it, it

required the whole diff machinery: FileDiff models, DiffSet,

repositories, the patching process, etc. That made it impractical for

any usage beyond showing diffs of files in a repository.
This change splits the heavy processing out into a RawDiffChunkGenerator

class, which can be used to generate chunks from any two strings.
DiffChunkGenerator subclasses RawDiffChunkGenerator, doing all the work

of handling interdiffs and generating suitable content from FileDiffs to

pass down to the RawDiffChunkGenerator functions.
Along with this, DiffOpcodeGenerator no longer needs FileDiffs as well.

It only ever needed the diff content stored on them, for interdiff

processing, so we just pass that directly in DiffChunkGenerator.

Browsed diffs and interdiffs, comparing them to versions without the path.

This covered all types of changes, including indentation. I didn't see any

differences in behavior.
Tested with raw strings, and got chunks out of them.
Unit tests pass.