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>

diff.c

@@ -2784,7 +2784,7 @@ static void show_stats(struct diffstat_t *data, struct diff_options *options)
 		else if (file->is_unmerged) {
 			strbuf_addf(&out, " %s%s%*s | %*s",
 				    prefix, name, padding, "",
-				    number_width, "Unmerged");
+				    number_width, "Unmerged\n");
 			emit_diff_symbol(options, DIFF_SYMBOL_STATS_LINE,
 					 out.buf, out.len, 0);
 			strbuf_reset(&out);

t/t4046-diff-unmerged.sh

@@ -86,4 +86,14 @@ test_expect_success 'diff-files -3' '
 	test_cmp diff-files-3.expect diff-files-3.actual
 '
 
+test_expect_success 'diff --stat' '
+	for path in $paths
+	do
+		echo " $path | Unmerged" || return 1
+	done >diff-stat.expect &&
+	echo " 0 files changed" >>diff-stat.expect &&
+	git diff --cached --stat >diff-stat.actual &&
+	test_cmp diff-stat.expect diff-stat.actual
+'
+
 test_done