t9001: avoid non-portable '\n' with sed

t9001 used a '\n' in a sed expression to split one line into two
lines.  Some versions of sed (/usr/bin/sed under Mac OS X) simply
ignore the '\' before the 'n', treating '\n' as 'n'.

As the test already requires perl as a prerequisite, use perl
instead of sed.

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