This introduces get_newline_for_type(), a utility function that takes

a LineEndings type value and returns newlines encoded as either ASCII

or the given encoding (with BOM stripped).
This logic was previously contained in DiffXReader, but is generally

useful, so it's being split out. That simplifies the DiffXReader code

as well, making some of the logic a lot more clear.

Unit tests pass on Python 2 and 3.