报错信息：

~/Documents/» brew install tree
Error: 
  homebrew-core is a shallow clone.
To `brew update`, first run:
  git -C /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core fetch --unshallow

解决方案

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

可能会因为网络原因报错，换几个时间点，多执行几次，执行期间显示如下：

其实就是把https://github.com/Homebrew/homebrew-core/部分下载下来重新更新本地仓库。

然后可以brew update验证一下是不是不会报错了

如果还有报错： homebrew-cask is a shallow clone.
类似的，可以使用

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

相对靠谱的内容：

• stack overflow：How to remove the shallow clone warning from HomeBrew
• HomeBrew - Error: homebrew-core is a shallow clone.

有风险，不建议采用

• [Solved] Mac Homebrew Error: homebrew-core is a shallow clone.
  
• 这一解决方案的实践：Mac homebrew报错Error: homebrew-core is a shallow clone.