t9402: sed -i is not portable

t9402: sed -i is not portable

On some systems sed allows the usage of e.g.
sed -i -e "s/line1/line2/" afile
to edit the file "in place".
Other systems don't allow that: one observed behaviour is that
sed -i -e "s/line1/line2/" afile
creates a backup file called afile-e, which breaks the test.
As sed -i is not part of POSIX, avoid it.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>