update_command() {
  local update_to_head=$1

  (
  cd $(asdf_dir)

  if [ "$update_to_head" = "--head" ]; then
    # Update to latest on the master branch
    git checkout master

    # Pull down the latest changes on master
    git pull origin master
    echo "Updated asdf to latest on the master branch"
  else
    # Update to latest release
    git fetch --tags || exit 1
    tag=$(git tag | sort_versions | sed '$!d') || exit 1

    # Update
    git checkout "$tag" || exit 1
    echo "Updated asdf to release $tag"
  fi
  )
}

# stolen from https://github.com/rbenv/ruby-build/pull/631/files#diff-fdcfb8a18714b33b07529b7d02b54f1dR942
sort_versions() {
    sed 'h; s/[+-]/./g; s/.p\([[:digit:]]\)/.z\1/; s/$/.z/; G; s/\n/ /' | \
        LC_ALL=C sort -t. -k 1,1 -k 2,2n -k 3,3n -k 4,4n -k 5,5n | awk '{print $2}'
}