In this case, it's a foreign Mercurial repository, not Git. And remember that users' builds can contain arbitrary custom shell scripts that perform "git clone"

In this case, it's a foreign Mercurial repository, not Git. And remember that users' builds can contain arbitrary custom shell scripts that perform "git clone" or "hg clone" commands. It could be very, very difficult for Github to build an automated system that meddles with the behavior of those scripts to introduce caching, while guaranteeing not to break anything.