For what it's worth, XLSX files aren't binary. They're just XML in a ZIP. They're not particularly nasty to diff once they're extracted - actually, I think they might do pretty well in Git.
The diff is actually not really straightforward - a sheet is two-dimensional and you have to work out column/row inserts/deletes (which themselves are intertwined) before you can even start looking at cell changes. So it's quite non-standard stuff.
