升级big sur 后，执行
brew update 时会有如下提示：
homebrew-core is a shallow clone.
homebrew-cask is a shallow clone.
To `brew update`, first run:
git -C /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core fetch --unshallow
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-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
git -C /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core fetch --unshallow 时，又提示
fatal: dumb http transport does not support shallow capabilities,
进入到 /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core 目录下
git remote -v 发现之前将 brew 换成了 阿里源，最终换成默认仓库解决问题
cd "$(brew --repo)/Library/Taps/homebrew/homebrew-core"
git remote set-url origin https://github.com/Homebrew/homebrew-core.git
cd "$(brew --repo)/Library/Taps/homebrew/homebrew-cask"
git remote set-url origin https://github.com/Homebrew/homebrew-cask.git
不知什么原因，fetch homebrew-core 那一步还是失败，最后索性删掉 homebrew-core 目录 从新 clone 一份，至此，
brew update 终于可以正常更新了
git clone https://github.com/Homebrew/homebrew-core.gitunshallow