[git] Why doesn't Git ignore my specified file?

What I did it to ignore the settings.php file successfully:

  1. git rm --cached sites/default/settings.php
  2. commit (up to here didn't work)
  3. manually deleted sites/default/settings.php (this did the trick)
  4. git add .
  5. commit (ignored successfully)

I think if there's the committed file on Git then ignore doesn't work as expected. Just delete the file and commit. Afterwards it'll ignore.