diff: fix regression with --stat and unmerged file

diff: fix regression with --stat and unmerged file

A regression was introduced in

  12fc4ad89e (diff.c: use utf8_strwidth() to count display width, 2022-09-14)

that causes missing newlines after "Unmerged" entries in `git diff
--cached --stat` output.

This problem affects v2.39.0-rc0 through v2.39.0.

Add the missing newline along with a new test to cover this
behavior.

Signed-off-by: Peter Grayson <pete@jpgrayson.net>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>