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>