t4012: Fix invalid call to dd(1)

t4012 assumed that it is okay to seek beyond the end of stdin. It is not,
and leads to an access violation (Windows speak for: segmentation fault)
with MinGW's dd.exe iff run in non-verbose mode.

Further, for some reason 'echo X | dd bs=1k seek=1' resulted in 2050 bytes
in this developer's setup instead of 1026.

So let's use a valid technique to generate 1024 NULs followed by X and a
line feed: a good ole' Perl script.

This patch is dedicated to Randal "Merlyn" Schwartz.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t4012-diff-binary.sh

@@ -113,7 +113,7 @@ 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 &&
+	perl -e "print \"\\0\" x 1024; print \"X\\n\";" > binfile &&
 	git add binfile &&
 	i=0 &&
 	while test $i -lt 10000; do