title: use git apply --whitespace=fix from command line to manually fix normal text files
body: <p>I have a number of text files and from time to time I start up a tool that removes the trailing white space. Is it possible to use <code>git apply --whitespace=fix</code> for that purpose (and get rid of the other tool I was using)?  </p>
  
  <p>I tried something like <code>git apply --whitespace=fix testfile.txt</code> but that did <strong>not</strong> work, 
  I guess it only works on patch files, but I want to use it with any kind of text files.</p>
  
  <p>I do not want to run the command on every check-out or commit (there are some answers doing that), I just want to run it manually from time to time and if I like the output, I will commit it, separately from changes to the content.</p>