cleanMerge derailed if editor reformats JSON file

[maxg]

Students using VS Code with auto-formatting enabled (e.g. from Prettier) and who don't realize they committed a reformat of the entire JSON file will see a merge conflict when the exercise expects an automatic merge, and the web page simply won't proceed to the next step.

Could fail with an error if a bad commit is detected or a merge conflict is detected.

Could disable autoformatting for JSON in .vscode/settings.json, something like:

"[json]": { "editor.formatOnSave": false }