lf_to_crlf_filter(): tell the caller we added "\n" when draining

lf_to_crlf_filter(): tell the caller we added "\n" when draining

This can only happen when the input size is multiple of the
buffer size of the cascade filter (16k) and ends with an LF,
but in such a case, the code forgot to tell the caller that
it added the "\n" it could not add during the last round.

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