title: Git: erase files from repository from previous commits
body: <p>I am on github. I set up a .gitignore file in which I specify files not to be added to future commits since I'm trying to lighten the cloning. Now I need to delete all those .class files for example from all the previous commits to share only the necessary sources, otherwise no matter my efforts it will always weight 50mb more or less. I had a look on <a href="http://cheat.errtheblog.com/s/git" rel="nofollow">http://cheat.errtheblog.com/s/git</a>
  I think I might need a composition of those command but I really don't wanna s***w up my work.
  thanks</p>