出现情景

Mac升级到新版本使用brew upgrade出现.

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!

原因:

因为更新浅克隆(shallow clones)是一个相当消耗资源的操作，所以github请求被限制了

解决方法:

删除homebrew-core后再更新

cd /usr/local/Homebrew/Library/Taps/homebrew

rm -rf homebrew-core

brew upgrade