Comment by b5n

  strict_ignore() {
    [[ -n "$1" ]] || exit 1
    local repo="${1%/}/"
    local ignore
    ignore=$(find "$repo" -path "$repo".git -prune -o -print \
               | sed "s|$repo||g; /^$/d" \
               | awk '{print "!"$0}' \
               | sort)
    printf "*\n%s\n" "$ignore"
  }