t4012: use 'printf' instead of 'dd' to generate a binary file

For some reason, 'echo X | dd bs=1k seek=1' creates a file with 2050 bytes
on Windows instead of the expected 1026 bytes, so that a test fails. Since
the actual contents of the file are irrelevant as long as there is at
least one zero byte so that the diff machinery recognizes it as binary,
use printf to generate it.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Alexander Strasser <eclipse7@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4012-diff-binary.sh

@@ -131,7 +131,7 @@ cat >expect <<EOF
 EOF
 
 test_expect_success 'diff --stat with binary files and big change count' '
-	echo X | dd of=binfile bs=1k seek=1 &&
+	printf "\01\00%1024d" 1 >binfile &&
 	git add binfile &&
 	i=0 &&
 	while test $i -lt 10000; do