执行brew update时出现了下面的错误
error: Not a valid ref: refs/remotes/origin/master
fatal: Could not resolve HEAD to a revision
Already up-to-date.
解决方法如下
cd /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core
git fetch --prune origin
git pull --rebase origin master
brew install
搞定了～