The "rot13-filter.pl" helper wrote its debug logs always to "rot13-filter.log".
Make this configurable by defining the log file as first parameter of
"rot13-filter.pl".
This is useful if "rot13-filter.pl" is configured multiple times similar to the
subsequent patch 'convert: add "status=delayed" to filter process protocol'.
Signed-off-by: Lars Schneider <larsxschneider@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>