VC Shuttle: Advice-Based Git Sync for Air-Gapped Emacs

I work on an air-gapped VM, which is to say, a virtual machine with no network access at all. You copy code in via USB or a shared folder, develop, and then you need to get that code back out to the host machine. Git is running inside the VM, of course, it's the sensible thing to do for version control, but git push wants a remote, and there is no remote. What you actually need is a file copy…