GIT_ROOT="$(git rev-parse --show-toplevel)"

find "$GIT_ROOT/.git/objects" -type f | \
while read -r file; do
    if echo "$file" | grep -Eq '\.idx$'; then
        git show-index < "$file" | awk '{print $2}'
    elif echo "$file" | grep -Eq '[0-9a-f]{38}$'; then
        echo "$(basename "$(dirname "$file")")$(basename "$file")"
    fi
done | \
while read -r hash; do
    if [ "$(git cat-file -t "$hash")" = commit ]; then
        echo "$hash"
    fi
done