title: Windows git difftool cannot parse extcmd path with spaces
body: <p>In Windows, from the Git MSYS command line, I would like to run this command:</p>
  
  <pre><code>git difftool --extcmd=/C/Program Files/Beyond Compare 4/Bcomp.exe -- file_to_diff.ext
  </code></pre>
  
  <p>I'm struggling to have the <code>extcmd</code> parameter parsed correctly.</p>