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.
i18n-ja: synchronized with
2478594b37c2
i18n-pt_BR: synchronized with
0a730d3c5aae