Apple's Swift programming language is on GitHub -- the full history AFAIK.

https://github.com/apple/swift

That depends. You don’t have any helpful commit logs or even changelogs. But if you arrange e.g. the XNU releases in the form of a reasonable graph and import them into git that way, you can get some telling diffs, on the command line at least. GitHub is not being very helpful here because it does not allow you to diff arbitrary tags and revisions from a repository. But GitHub is still a place where people find and look for code.

Those thoughts prompted me to create https://github.com/epipping/xnu-kernel-sources-x86 (and its cousin https://github.com/epipping/xnu-kernel-sources-ppc). Maybe you find it help (please take a look at the wiki)

To my knowledge, they haven't either (outside what is on Github already). The point being it would be great if they did.