My company stores Word documents in SVN, and accesses them via TortoiseSVN.
Tortoise uses Word's built in change tracking function to show you a "diff" of two revisions.
This works really well, but requires Windows and Word.
Edit:
You could probably get this working with git too. If you install TortoiseSVN, then look in %PROGRAMFILES%\TortoiseSVN\Diff-Scripts\, you'll see what tortoise is doing.
If you're using git, I assume you're 1337 enough to hack it to work for you :)