Quantifying the amount of change in a git diff?

wdiff does word-by-word comparison. Git can be configured to use an external program to do the diffing. Based on those two facts and this blog post, the following should do roughly what you want.

Create a script to ignore most of the unnecessary arguments that git-diff provides and pass them to wdiff. Save the following as ~/wdiff.py or something similar and make it executable.

#!/usr/bin/python

import sys
import os

os.system('wdiff -s3 "%s" "%s"' % (sys.argv[2], sys.argv[5]))

Tell git to use it.

git config --global diff.external ~/wdiff.py
git diff filename