Shell commands to illustrate issue with git clean filter

gistfile1.txt

cd ~/tmp
mkdir cleansmudge
cd cleansmudge
git init
echo content > file.txt
echo "*.txt filter=autoformat" > .git/info/attributes
echo "[filter \"autoformat\"]
 clean = date >> ~/tmp/filteroutput.log
 smudge = cat" >> .git/config
# After setting the filter, run 'tail -f ~/tmp/filteroutput.log' in another term.
git add file.txt 
git commit -m"Initial commit"
git checkout -b b1
echo row2 >> file.txt 
less file.txt

Result: at every new prompt, two new dates appear in ~/tmp/filteroutput.log. I wonder if the git part of my prompt is the issue.