According to the Xcode 9 release notes:
"Xcode Server no longer includes hosting Git repositories. To continue using repositories that were set up in macOS Server, configure a new remote Git server and push your code to that remote. After upgrading to Xcode 9, any existing repositories are archived in /Library/Developer/XcodeServer/HostedRepositories."
If you don't want to use GitHub (or, I assume, any of the other fine SCM repository services out there), then you'll have to run your own, and take on all the headaches of running and securing your server, I guess.