Earlier quoted context omitted.
GP isn't saying GitHub should copy the entire repo, only that there should be some indication that the code you're looking at isn't the repo owner's (despite being committed in their name and on a repo they "control"). I don't see what optimization requires that. They already keep track of e.g. me pushing up someone else's commit after a rebase -- it indicates that I pushed but the commit originally came from someone…
> I don't see what optimization requires that. From a single commit ID you cannot tell which repo it came from. A "repo" is just a tree of commits.
(You might argue that determining what refs contain a commit is potentially expensive, perhaps, but GitHub already does this, so I'd argue that it's not that expensive.)