Mac使用Command Line Tools不用装Xcode
亲自测试过（请注意看错误提示）
-> git
xcrun: error: active developer path
("/Applications/Xcode.app/Contents/Developer")
does not exist, use `xcode-select --switch path/to/Xcode.app`
to specify the Xcode that you wish to use for command line
developer tools (or see `man xcode-select`)
解决办法
xcode-select --install # 单独安装CommandLineTools，不需要Xcode
sudo xcode-select --switch /Library/Developer/CommandLineTools # 指定路径
(第二部可以不运行)
再执行 git，显示OK了。
-> git
usage: git [--version] [--help] [-C <path>] [-c name=value]
[--exec-path[=<path>]] [--html-path] [--man-path] [--info-path]
[-p | --paginate | --no-pager] [--no-replace-objects] [--bare]
[--git-dir=<path>] [--work-tree=<path>] [--namespace=<name>]
<command> [<args>]
