text: Title: Add option to run the changed files from the current branch
  Body: Let's say you are working on a feature branch and would like to run all the tests that were changed when compared with the master. To check it, you can run: `git diff --name-only master`.
  
  Would be nice to have this option on _picked_. I'm wondering if something like `pytest --picked=branch` would be the best way to go.