?问题描述：
在进行brew update时，发生错误，如下：
% brew update
fatal: Could not resolve HEAD to a revision
Already up-to-date.
解决办法：
思路：
1）找到出错的homebrew的目录；
2） 进入到该目录，执行fetch 和?pull命令。
1）运行“brew update --verbose”，找到出错的homebrew的目录：
% brew update --verbose
Checking if we need to fetch /usr/local/Homebrew...
Checking if we need to fetch /usr/local/Homebrew/Library/Taps/homebrew/homebrew-cask...
Checking if we need to fetch /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core...
Fetching /usr/local/Homebrew...
Updating /usr/local/Homebrew...
Branch 'master' set up to track remote branch 'master' from 'origin'.
Switched to and reset branch 'master'
Your branch is up to date with 'origin/master'.
Switched to and reset branch 'stable'
Current branch stable is up to date.
Updating /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core...
fatal: Could not resolve HEAD to a revision
Already up-to-date.
2）进入到出错的目录，然后先fetch，后pull，如下：
cd /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core
% pwd
/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core
% git fetch --prune origin
% git pull --rebase origin master
?From https://github.com/Homebrew/homebrew-core
* branch? ? ? ? ? ? ? ? ? ? master ? ? -> FETCH_HEAD
Updating files: 100% (6435/6435), done.
如下图：
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