git commit --author "Bob <bob@example.com>" -m "preserve: a change by bob" file1 &&
echo "username-unknown: a change by charlie" >>file1 &&
git commit --author "Charlie <charlie@example.com>" -m "preserve: a change by charlie" file1 &&
git commit --author "Bob <bob@example.com>" -m "preserve: a change by bob" file1 &&
echo "username-unknown: a change by charlie" >>file1 &&
git commit --author "Charlie <charlie@example.com>" -m "preserve: a change by charlie" file1 &&