send-email: multiedit is a boolean config option

send-email: multiedit is a boolean config option

The sendemail.multiedit variable is meant to be a boolean.
However, it is not marked as such in the code, which means
we store its value literally. Thus in the do_edit function,
perl ends up coercing it to a boolean value according to
perl rules, not git rules. This works for "0", but "false",
"no", or "off" will erroneously be interpreted as true.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>