t4012: Actually quote the sed script

t4012: Actually quote the sed script

The nested quoting is not needed in this cases, thus the previous
version did work just fine. Nevertheless the usage is misleading,
so just achieve nested quoting by using double quotes instead. Lower
the probability of breakage in the future and make the code easier
to read.

NOTE: Just dropping the single quotes around the sed arguments would
      have also been possible.

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