>I’ve never tried this, so take it with a heap of salt, but you might have better luck with the diffing if you developed a process that unzipped the file contents into your repo after you’re done editing it, and then zipped the contents back up with the right extension when you want to edit it again.

I have a current project where one of the other developers has tried this. It might work for a solo project, but with multiple developers on Windows (Excel) and Linux (Open Office), I find that Open Office feels a need to continually fuck with unrelated fields. Editing a single field in an xslx file results in changes in a dozen files in the unpacked version. Making any more complicated changes results in a diff that's impossible to manage.