Download and install the docx2txt converter from http://docx2txt.sourceforge.net/
wget -O doc2txt.tar.gz http://docx2txt.cvs.sourceforge.net/viewvc/docx2txt/?view=tar
tar zxf docx2txt.tar.gz
cd docx2txt/docx2txt/
sudo make
(simply follow the INSTALL instructions for your platform)
Then make a small wrapper script to make docx2txt output to STDOUT
echo '#!/bin/bash
docx2txt.pl "$1" -' > /usr/local/bin/docx2txt
chmod +x /usr/local/bin/docx2txt
echo "*.docx diff=wordx" >> .gitattributes
git config diff.wordx.textconv docx2txt
Use .git/info/attributes
if the setting should not be committed with the project.
echo "*.docx diff=word" >> .gitattributes
git config diff.word.textconv strings