t/t8003-blame-corner-cases.sh: Use here documents

Somehow, this test was using:

{
echo A
echo B
} > file

block to feed file contents. This changes those to the form most common
in git test scripts:

cat >file <<-\EOF
A
B
EOF

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>