I’ve been struggling with this problem for a while, and just discovered (for me) a perfect solution:
$ git config --global diff.tool vimdiff # or merge.tool to get merging too!
$ git difftool commit1 commit2
git difftool takes the same arguments as git diff would, but runs a diff program of your choice instead of the built-in GNU diff. So pick a multibyte-aware diff (in my case, vim in diff mode) and just use git difftool instead of git diff.
Find “difftool” too long to type? No problem:
$ git config --global alias.dt difftool
$ git dt commit1 commit2
Git rocks.