fix_email_parse2.diff slightly changes behavior. See my comments on Rietveld.

As for fix_prepending2.diff, could you please provide any benchmark results?

And there is yet one bug in current code. str.splitlines() splits a string not only breaking it at '\r', '\n' or '\r\n', but at any Unicode line break character (e.g. '\x85', '\u2028', etc). And when a chunk ends with such line break character, it will not break a line. Definitely something should fixed: either lines should be broken only at '\r', '\n' or '\r\n', or other line break characters should be handled correctly when they happen at the end of the chunk. What would you say about this, David?