Fix for SF bug [ #471928 ] global made w/nested list comprehensions

. Initially I was going to just rip out the bits of this that fixed this
  bug, but the rest of the code looks (after a fair amount of staring at
  it) like it's ok - variable renames, that sort of thing.
  flames and "hey, no way!" to me, or to python-dev.
  It felt safer to just go with the full patch, rather than butchering
  it.