title: Create new pull request from fork without having commits of the previous fork
body: <p>So i've made a few commits to my fork of someone else's repo, and then submitted a pull request for the same. My old pull request hasn't been merged yet, and now i need to submit a new pull request without the previous commits, but with new changes that i'm going to make to my fork (i need my old pull request with its commits to stay, too). How can i do this?</p>