Who hasn't commited debug code by mistake? It's only normal to forget to remove an #include we added only to test some stuff. Luckily it's easy to tell git that we don't want to commit any changes with a certain string.
On any (git) repo you'll find a .git/hooks folder; add this script in .git/hooks/pre-commit (and don't forget to chmod +x it):
#!/bin/sh
if [ 0 != git diff | grep "STOPCOMMIT" | wc -l ]; then
echo "Error: STOPCOMMIT found, remove it before commiting";
git diff
exit 1
fi
Now git will check your commits and stop them if they contain the STOPCOMMIT string. Now you can add all the debug changes you want, as long as you add a //STOPCOMMIT after them you'll never end up commiting them by mistake.