t4015: use test_write_lines()

Instead of rolling our own method to write out some lines into a file,
use the existing test_write_lines().

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4015-diff-whitespace.sh

@@ -771,9 +771,9 @@ test_expect_success 'checkdiff detects new trailing blank lines (1)' '
 '
 
 test_expect_success 'checkdiff detects new trailing blank lines (2)' '
-	{ echo a; echo b; echo; echo; } >x &&
+	test_write_lines a b "" "" >x &&
 	git add x &&
-	{ echo a; echo; echo; echo; echo; } >x &&
+	test_write_lines a "" "" "" "" >x &&
 	test_must_fail git diff --check >check &&
 	grep "new blank line" check
 '