diff: highlight and error out on incomplete lines

diff: highlight and error out on incomplete lines

Teach "git diff" to highlight "\ No newline at end of file" message
as a whitespace error when incomplete-line whitespace error class is
in effect.  Thanks to the previous refactoring of complete rewrite
code path, we can do this at a single place.

Unlike whitespace errors in the payload where we need to annotate in
line, possibly using colors, the line that has whitespace problems,
we have a dedicated line already that can serve as the error
message, so paint it as a whitespace error message.

Also teach "git diff --check" to notice incomplete lines as
whitespace errors and report when incomplete-line whitespace error
class is in effect.

Signed-off-by: Junio C Hamano <gitster@pobox.com>