title: how can I untrack files in a branch that is not my current branch
body: <p>I have 2 git branches 'Master' and 'Work'.</p>
  
  <p>I had been working on my 'Work' Branch for a while. In work I untracked all my *.png files and added *.png to my gitignore.</p>
  
  <p>The master branch is still tracking *.png.</p>
  
  <p>Now I can't checkout my master branch because git is complaining that my untracked files would be over written by checkout. </p>
  
  <p>I can't checkout my master branch and merge my work branch into it.</p>
  
  <p>How can I tell my master branch to stop tracking *.png.</p>
  
  <p>I know you usually use rm -r --cached to remove files but I only know how to do that from the current branch.</p>