mac brew update時にエラー「Error: homebrew-cask is a shallow clone. To brew update first run:」が発生した場合の対処法

Error: homebrew-cask is a shallow clone. To `brew update` first run:
  git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-cask" fetch --unshallow
This restriction has been made on GitHub's request because updating shallow
clones is an extremely expensive operation due to the tree layout and traffic of
Homebrew/homebrew-cask. We don't do this for you automatically to avoid
repeatedly performing an expensive unshallow operation in CI systems (which
should instead be fixed to not use shallow clones). Sorry for the inconvenience!

git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-cask" fetch --unshallow