（转自：http://c.n-di.com/2017/07/17/1827.html）
更新brew，报错；
Error: Could not link:
/usr/local/etc/bash_completion.d/brew
Please delete these paths and run brew update.
Error: Could not link:
/usr/local/share/zsh/site-functions/_brew
Please delete these paths and run brew update.
Error: Could not link:
/usr/local/share/man/man1/brew.1
Please delete these paths and run brew update.
Error: Could not link:
/usr/local/share/doc/homebrew
Please delete these paths and run brew update.
解决办法
rm -rf /usr/local/etc/bash_completion.d/brew
rm -rf /usr/local/share/zsh/site-functions/_brew
rm -rf /usr/local/share/man/man1/brew.1
rm -rf /usr/local/share/doc/homebrew
brew update
