tests: do not pass iso8859-1 encoded parameter

tests: do not pass iso8859-1 encoded parameter

git commit -m with some iso8859-1 encoded stuff is doomed to fail in MinGW,
because Windows don't let you pass encoded bytes to a process (CreateProcessW
always takes a UTF-16LE encoded string).

It is safe to pass the iso8859-1 message using a file or a pipe.

Thanks-to: Karsten Blees <blees@dcon.de>
Author: Stepan Kasal <kasal@ucw.cz>
Signed-off-by: Stepan Kasal <kasal@ucw.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>