The SVN repository that Jeff mentioned is currently the official "source" to use.
The statement "Code is being brought from SVN into GitHub in multiple repositories" should be indicative enough of an ongoing process, which entails separating the projects out of a monolithic structure, migrating/cleaning up their history (in some cases maybe dropping that) and figuring out a framework to manage the public code, private code (including Enterprise Edition specific maintenance branches for otherwise public code) and a (hopefully) improved community contribution process. Obviously this cannot be done over night and Alfresco also has to divide its attention among all the ongoing support / preparation for the next version.
Use the SVN for now, and keep checking on this platform for any future announcements from Alfresco regarding changes to source access and contribution processes. You can always check the Source Code document to see which projects are already available in GitHub / GitLab.