title: How an I make `git ls-files` work in subdirectories?
body: <p>I want to run <code>git ls-file --modified</code> to obtain a list of files with unstaged changes. I was surprised to find that it works in the root directory, but not in any subdirectory, where it only prints files from the subdirectory (in contrast to <code>git status</code>, for instance).</p>
  
  <p>Steps to reproduce:</p>
  
  <pre><code>mkdir repo
  cd repo
  git init
  touch file.txt
  git add file.txt 
  mkdir subdirectory
  echo "foo" &gt;&gt; file.txt    # file now has unstaged changes
  git ls-files --modified   # This prints 'file.txt'
  cd subdirectory/
  git ls-files --modified   # This prints nothing
  </code></pre>
  
  <p>How can I change git's behaviour here?</p>