It would be great if users can edit and save revisions for WordPress core, plugin and theme files. We would have to create another revision copy in another location, delegate this copy instead of the original one.
We believe this can be achieved for CSS and JavaScript files and we have many workarounds and solutions to get this to work for PHP, however we really need to investigate this further to see if it’s possible to do.
Important Note
It is not a good idea to update the real file on disk, as this is a security risk and might fail under many servers. It also might destroy the site.