Assuming a modicum of technical sophistication on behalf of the legal people (which may be an invalid assumption), I think the fact that git/github is centered on the forking/sharing of code with an explicit graph to walk would just incur too much liability for them because it is obvious (in a technical sense) where the code has gone.

You don't have to do any forensics in the sense of your youtube example, you just have to follow the graph.

Whether that it is a legally compelling point, I have no idea.