title: git diff tmp file invalid on windows when using external program on Windows 7
body: <p>I'm following the docs from the <a href="http://git-scm.com/book/en/Customizing-Git-Git-Configuration" rel="nofollow">Git configuration page</a> and trying to apply this to my Windows 7 environment. When I run say, "git diff file0" I get an error in P4Merge:</p>
  
  <pre><code>Errors: '/tmp/cH9ccM_file0' is (or points to) an invalid file.
  </code></pre>
  
  <p>This is a true statement. I do not have a directory called "/tmp". I've checked in C:\tmp, C:\cygwin\tmp, and %PROGRAMFILES%\Git\tmp. </p>
  
  <p>Any ideas on how to change this directory to something I do have?</p>
  
  <p>EDIT:</p>
  
  <p>Additional info. Ultimately, I'm trying to get WinMerge (or any external program) to work. I chose to use P4Merge earlier, because I couldn't get WinMerge to work, so I decided to go with the program that was used in the aforementioned article. </p>
  
  <p>Below is the relevant part of my .gitconfig:</p>
  
  <pre><code>[merge]
  tool = extMerge
  [mergetool "extMerge"]
  cmd = extMerge \"$BASE\" \"$LOCAL\" \"$REMOTE\" \"$MERGED\"
  trustExitCode = false
  [diff]
  external = extDiff
  </code></pre>
  
  <p>extDiff script:</p>
  
  <pre><code>#!/usr/bin/sh
  # extDiff
  # Wrapper for using an external diff tool in Git
  echo "File 1: $2"
  echo "File 2: $5"
  /usr/local/bin/extMerge "$2" "$5"
  </code></pre>
  
  <p>extMerge script:</p>
  
  <pre><code>#!/usr/bin/sh
  echo $*
  /c/Program\ Files\ \(x86\)/WinMerge/WinMergeU.exe $*
  </code></pre>
  
  <p>So if I change a file called index.html, and run git diff, this is the result:</p>
  
  <pre><code>File 1: /tmp/XfGpOC_index.html
  File 2: index.html
  </code></pre>