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>