Change send-email to use Perl's catfile() function instead of
"$dir/$file". If send-email is given a $dir that ends with a / we'll
end up printing a double slashed path like "dir//mtfnpy.patch".
This doesn't cause any problems since Perl's IO layer will handle it,
but it looks ugly.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>