title: Git: split pull request into smaller PR's based upon the new directories in the pull request
body: <p>I have a monolith of a feature branch. Instead of one massive PR into master I would like to split it up into 3 prs. </p>
  
  <p>Ideally I want to pull out some standalone code from the feature branch into a PR by itself. This code is in a new directory and isn't called into yet so it would be a relatively safe PR. However instead of just copying the directory and creating a single commit and PR I would like to retain all the commit history of the changes for the new PR.</p>
  
  <p>Is this possible using Git?  I've looked into filter-branch but it seems like that is for splitting a repo into two, not for splitting a diff of changes into two (if that makes sense).</p>