The idea for the attached patch comes from the QNX development team. In their measurements, replacing the re.split-plus-line-reassembly code in BufferedSubFile with str.splitlines provided a 30% reduction in email parsing time. The code is also a lot more readable, which is a plus. The patch is simple enough, and the improvement is large enough, that I'd like to apply this to all active branches.
This is a performance enhancement. Out of question for 3.2. Python 3.3 is in beta now, and this would be considered a new feature, but I think it is pretty safe to apply. I am +1 to applying it. Personally I would apply it to 2.7 too, but current official position is "bug fixes only". We have rejected performance improvements for 2.7 in the past, for this reason.
