解决方法

删除homebrew-core后更新即可

cd /usr/local/Homebrew/Library/Taps/homebrew
rm -rf homebrew-core
brew upgrade

问题解析

Mac更新完系统后使用homebrew就报错：

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 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!

使用brew upgrade后会重新更新homebrew-core