Let's hope that three time's a charm...

Tim discovered another "bug" in my get_line() code: while the comments
said that n<0 was invalid, it was in fact still called with n<0 (when
PyFile_GetLine() was called with n<0).  In that case fortunately
executed the same code as for n==0.

Changed the comment to admit this fact, and changed Tim's MS speed
hack code to use 'n <= 0' as the criteria for the speed hack.