Collaborative editing and persistently tracking changes

[David Reitter <david.reitter@gmail.com>]

How would one track changes in Emacs and mark them up directly in an  
Emacs buffer?

I often collaborate with people on documents and it's useful to see  
what changes they've made.  Many of my collaborators will want to use  
Word, if only for its nice change-tracking feature.

I would need a combination of highlight-changes-mode and a way to save  
and load the change history of a buffer into a file, just like MS Word  
does it (with the new open docx format).

One option would be to use some sort of repository, or to store  
versions of files before and after the changes and use M-x diff.  But  
that doesn't do a good job of tracking in-line changes (in times of  
soft-wrapping, paragraphs are file lines!), and obviously I can't  
force my collaborators to use extra tools.  The added pain of having  
to manage multiple file versions is also considerable, and there's  
limited extra benefit in versioning repositories with, say,  
manuscripts.  You hardly ever want to inspect older history, or go  
back to a previous version.  (Having separate branches might work  
well, but it's hard to get collaborators to commit to this technical  
overhead. And again, even git's merge algorithm is line-based.)

All in all, I'm missing good Emacs support for non-concurrent  
(asynchronous) real-time editing.  Suggestions would be welcome.

- David Reitter