/usr/bin/git-ignore is in git-extras 1.9.1-2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49  | #!/bin/bash
function show_contents {
  test -f "$2" && echo "$1 gitignore: $2" && cat "$2"
}
function show_global {
  show_contents Global `git config --global core.excludesfile`
}
function add_global {
  add_patterns `git config --global core.excludesfile` "$@"
}
function show_local {
  show_contents Local .gitignore
}
function add_local {
  add_patterns .gitignore "$@"
}
function add_patterns {
  echo "Adding pattern(s) to: $1"
  for pattern in "${@:2}"; do
    echo "... adding '$pattern'"
    (test -f "$1" && test "$pattern" && grep -q "$pattern" "$1") || echo "$pattern" >> "$1"
  done
}
if test $# -eq 0; then
   show_global
   echo "---------------------------------"
   show_local
else
  case "$1" in
    -l|--local)
      test $# -gt 1 && add_local "${@:2}" && echo
      show_local
      ;;
    -g|--global)
      test $# -gt 1 && add_global "${@:2}" && echo
      show_global
      ;;
    *)
      add_local "$@"
      ;;
  esac
fi
 |