China, GitHub and the man-in-the-middle (Greatfire)

It's not, but it goes a long way from nothing. Github is probably large and important enough (certainly after this) that the Chromium devs could ship with their certificate pinned, as they do for Tor and Twitter. By protecting the large web sites, any blanket MITM would also be discovered. If you are a developer and have access to out-of-band communications, perhaps it is worthwhile manually pinning the sites important to you.