title: Force git to update .gitignore
body: <p>I have a <code>.gitignore</code> file, and it's ignoring some files. I have updated the <code>.gitignore</code> file (removed some filenames and added some filenames). This is not reflected in <code>git status</code>. How can I force git to update these changes, so that track files which are not tracked before and vice versa. </p>
  
  <p>I have tried this <a href="https://stackoverflow.com/q/1139762/4576114">question</a>, still all of my files are not tracked (according to my updated <code>.gitignore</code>). (In simple, how can I force git to retract files once <code>.gitignore</code> is updated or deleted). </p>