Sorry for the late reply, but the simpler solution mentioned here of using git-diff output in code listings as-is should be possible already (just the simple rendering with green/red lines and emphasized headers).
The more advanced rendering with syntax highlighting code within diffs or utilizing our special diff format for tutorial code would require some coordinated with the DocC compiler.
I did notice with a quick test that the renderer doesn't have the right CSS to map the green/red colors to the appropriate tokens though, so I can create a quick issue and address that.