cleanup: use the deque type where appropriate There have been quite a few places where we pop elements off the front of a list. This can turn O(n) algorithms into something more like O(n**2). Python has provided a deque type that can do this efficiently since at least 2.4. As an example of the difference a deque can make, it improves perfancestors performance on a Linux repo from 0.50 seconds to 0.36.