brew update 更新时报错_to `brew update`, first run:

在 mac 下执行 brew update 时，报如下错误：

Error: 
  homebrew-core is a shallow clone.
  homebrew-cask is a shallow clone.
To `brew update`, first run:
  git -C /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core fetch --unshallow
  git -C /usr/local/Homebrew/Library/Taps/homebrew/homebrew-cask fetch --unshallow
These commands may take a few minutes to run due to the large size of the repositories.
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!

提示执行如下命令：

git -C /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core fetch --unshallow
git -C /usr/local/Homebrew/Library/Taps/homebrew/homebrew-cask fetch --unshallow

但试了几次都没有成功，报错如下：

git -C /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core fetch --unshallow
remote: Enumerating objects: 365476, done.
remote: Counting objects: 100% (365455/365455), done.
remote: Compressing objects: 100% (147319/147319), done.
fatal: The remote end hung up unexpectedly12 MiB | 9.00 KiB/s     
fatal: early EOF
fatal: index-pack failed

需要换到国内的镜像，执行如下命令：

## 更新 homebrew-cask（如果提示 No such file or directory 可以先跳过前三个命令）
cd "$(brew --repo)"/Library/Taps/homebrew/homebrew-cask
 
# 更换源
git remote set-url origin https://mirrors.ustc.edu.cn/homebrew-cask.git
 
# 更新
git fetch --unshallow 
 
## 更新 homebrew-core
cd "$(brew --repo)"/Library/Taps/homebrew/homebrew-core
 
# 更换源
git remote set-url origin https://mirrors.ustc.edu.cn/homebrew-core.git
# 更新
git fetch --unshallow 

常用的国内镜像有（上面的不行就多试几个）：

https://mirrors.ustc.edu.cn/homebrew-core.git
https://mirrors.aliyun.com/homebrew/homebrew-core.git
https://mirrors.tuna.tsinghua.edu.cn/git/homebrew/homebrew-core.git

再执行brew update就可以了。