tests: use test_write_lines() to generate line-oriented output

Take advantage of test_write_lines() to generate line-oriented output
rather than using for-loops or a series of `echo` commands. Not only is
test_write_lines() a natural fit for such a task, but there is less
opportunity for a broken &&-chain.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4126-apply-empty.sh

@@ -9,10 +9,7 @@ test_expect_success setup '
 	git add empty &&
 	test_tick &&
 	git commit -m initial &&
-	for i in a b c d e
-	do
-		echo $i
-	done >empty &&
+	test_write_lines a b c d e >empty &&
 	cat empty >expect &&
 	git diff |
 	sed -e "/^diff --git/d" \