brew/Library/Homebrew/cmd/update-reset.sh

#:  * `update-reset` [<repository> ...]
#:
#:  Fetch and reset Homebrew and all tap repositories (or any specified <repository>) using `git`(1) to their latest `origin/HEAD`.
#:
#:  *Note:* this will destroy all your uncommitted or committed changes.

homebrew-update-reset() {
  local DIR
  local -a REPOS=()

  for option in "$@"
  do
    case "${option}" in
      -\? | -h | --help | --usage) brew help update-reset; exit $? ;;
      --debug)                     HOMEBREW_DEBUG=1 ;;
      -*)
        [[ "${option}" == *d* ]] && HOMEBREW_DEBUG=1
        ;;
      *)
        REPOS+=("${option}")
        ;;
    esac
  done

  if [[ -n "${HOMEBREW_DEBUG}" ]]
  then
    set -x
  fi

  if [[ -z "${REPOS[*]}" ]]
  then
    REPOS+=("${HOMEBREW_REPOSITORY}" "${HOMEBREW_LIBRARY}"/Taps/*/*)
  fi

  for DIR in "${REPOS[@]}"
  do
    [[ -d "${DIR}/.git" ]] || continue
    ohai "Fetching ${DIR}..."
    git -C "${DIR}" fetch --force --tags origin
    git -C "${DIR}" remote set-head origin --auto >/dev/null
    echo

    ohai "Resetting ${DIR}..."
    head="$(git -C "${DIR}" symbolic-ref refs/remotes/origin/HEAD)"
    head="${head#refs/remotes/origin/}"
    git -C "${DIR}" checkout --force -B "${head}" origin/HEAD
    echo
  done
}