I just got hit with this issue. There doesn't seem to be any information on DigiCert's site or Github's.

edit: For some reason, deleting the expired DigiCert certificate from Keychain (and restarting Chrome) allowed it to find a valid chain to the Github certificate. I would recommend doing this if you want to get to Github without turning off SSL.

edit2: (Or they just fixed it and I restarted Chrome.) Can anyone confirm that it works now (without deleting the Intermediate Cert)?