为什么我不能用 Windows 版本的 Git 覆盖 mkdir？

Question

我已安装适用于 Windows 的 Git，并选中了“覆盖本机命令”选项。我已检查我的 PATH：

C:\Users\Domenic>echo %PATH%
C:\Program Files\Git\usr\bin;C:\Program Files\Git\cmd;C:\Program Files\nodejs\;C:\Windows\system32;C:\Windows;C:\python2;C:\python2\Scripts;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\ProgramData\Oracle\Java\javapath;C:\Program Files\Intel\iCLS Client\;C:\Program Files (x86)\Intel\iCLS Client\;C:\Program Files\Intel\Intel(R) Management Engine Components\DAL;C:\Program Files\Intel\Intel(R) Management Engine Components\IPT;C:\Program Files (x86)\Intel\Intel(R) Management Engine Components\DAL;C:\Program Files (x86)\Intel\Intel(R) Management Engine Components\IPT;C:\Program Files (x86)\GitExtensions\;C:\Program Files\LLVM\bin;C:\depot_tools;C:\Program Files (x86)\Windows Kits\10\Windows Performance Toolkit\;C:\Ruby21-x64\bin;C:\ProgramData\nodist\bin;C:\ruby200\bin;C:\Chocolatey\bin;C:\Program Files\kdiff3;C:\Program Files (x86)\Microsoft VS Code\bin;C:\Program Files\Docker Toolbox;C:\Users\Domenic\AppData\Roaming\npm;C:\Users\Domenic\Dropbox\PATH

（如果您不想滚动那么多，只需注意这C:\Program Files\Git\usr\bin是第一个）

我已经检查过where：

C:\Users\Domenic>where mkdir
C:\Program Files\Git\usr\bin\mkdir.exe

当我直接运行它时，它运行良好：

C:\Users\Domenic>"C:\Program Files\Git\usr\bin\mkdir.exe" --help
Usage: /usr/bin/mkdir [OPTION]... DIRECTORY...
Create the DIRECTORY(ies), if they do not already exist.

Mandatory arguments to long options are mandatory for short options too.
  -m, --mode=MODE   set file mode (as in chmod), not a=rwx - umask
(... etc ...)

但是，当我尝试mkdir从命令行运行时，它总是返回到 Windows 的内置命令行：

C:\Users\Domenic>mkdir --help
A subdirectory or file --help already exists.

C:\Users\Domenic>mkdir /?
Creates a directory.

MKDIR [drive:]path
(... etc ...)

为什么我不能覆盖 mkdir？

Windows 10，适用于 Windows 的 Git 2.7.3