GitHub for Windows doesn't put Git in the PATH by default. If you'd like your Node.js command prompt to have the git command available by default, simply edit your nodevars.bat file. By default, this is in C:\Program Files\nodejs\. You will have to run your text editor in an administrative context for this to work.
Replace this line in your nodevars.bat file:
set PATH=%APPDATA%\npm;%~dp0;%PATH%
With these thre lines:
for /F %%A in ('"dir /s /b /OD %userprofile%\appdata\local\git.exe"') do set gitPath=%%A\..