/usr/bin/git-release 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  | #!/usr/bin/env sh
hook() {
  local hook=.git/hooks/$1.sh
  if test -f $hook; then
    echo "... $1"
    . $hook
  fi
}
if test $# -gt 0; then
  hook pre-release
  echo "... releasing $1"
  git commit -a -m "Release $1"
  git tag $1 -a -m "Release $1" \
    && git push $2 \
    && git push $2 --tags \
    && hook post-release \
    && echo "... complete"
else
  echo "tag required" 1>&2 && exit 1
fi
 |