Although the option to disconnect from a Git repo has now been added, after restoring a previous version from within PowerApps, when I try to open it for editing Studio reconnects the app to the repository and automatically loads the latest version from the repo, making the restore feature useless.
It's particularly unhelpful in my situation as there is a PowerApps version that never made it (due to a bug) into the repo. While I can restore and publish it from PowerApps, I can't edit it in PowerApps Studio for the above reason.