Use git / diff when edits are made

[itsrachelfish]

If two people edit the page at the same time, it's easy to accidentally overwrite someone's work. The edit page should keep track of the place in history that a user is currently editing from so that a diff / automatic merge can be made when other people make edits at the same time.

If there is a merge conflict, the user would have to be shown a page where they can fix it before saving, otherwise their edit would be discarded.