title: update a virtualenv to match requirements.txt
body: <p>Currently, when our project's requirements.txt is updated, we re-build our virtualenv from scratch to ensure consistent results. The problem is that our requirements.txt is quite lengthy, and updates to it generally only touch a single package.</p>
  
  <p>Is there any system that compares a virtualenv to a requirements.txt and does the minimal amount of changes to make them match?</p>
  
  <p>I could possibly write this myself if I could get pip to tell me which versions it <em>would</em> install given a requirements.txt, but I don't see such an option.</p>