Add a utility function for returning encoded newlines for a type.
Review Request #11742 — Created July 23, 2021 and submitted
get_newline_for_type(), a utility function that takes
LineEndingstype 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
as well, making some of the logic a lot more clear.
Unit tests pass on Python 2 and 3.