brew updateしようとしたらhomebrew-core is a shallow clone.と言われてできない

> brew update      
Error: 
  homebrew-core is a shallow clone.
To `brew update`, first run:
  git -C /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core fetch --unshallow
This command may take a few minutes to run due to the large size of the repository.
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-core and 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!

gotohayato.com

まあつまり書いてあるとおりのことすればよい。

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

これまた書いてあるとおり、しばらく時間かかるのでしばらく放置しておこう。なにも読んでなくて1回キャンセルしちゃった。