I couldn't help but notice the actual logo wasn't one of the download options. Although inspecting the element tells us that the original logo is at https://a248.e.akamai.net/assets.github.com/images/modules/h...

I'm not entirely sure what Github was trying to accomplish with this, other than being a neat trick.

EDIT: https://a248.e.akamai.net/assets.github.com/images/modules/h... for the non-hover logo. ;)