On Windows you can just use TortoiseGIT, it can do diff and even merge by calling Word's internal compare tools. I can attest that diff works fine (differences

On Windows you can just use TortoiseGIT, it can do diff and even merge by calling Word's internal compare tools. I can attest that diff works fine (differences show up as if you had used track changes within word), but I haven't had occasion to try merging Word documents with TortoiseGIT yet. The same functionality was already available in TortoiseSVN.