send-email: fix threaded mails without chain-reply-to

send-email: fix threaded mails without chain-reply-to

An earlier commit 15da108 ("send-email: 'References:' should only
reference what is sent", 2009-04-13) broke logic to set up threading
information for the next message by rewriting "!" to "not" without
understanding the precedence rules of the language.

Namely,

    ! defined $reply_to || length($reply_to) == 0

was changed to

    not defined $reply_to || length($reply_to) == 0

which is

    not (defined $reply_to || length($reply_to) == 0)

and different from what was intended, which is

    (not defined $reply_to) || (length($reply_to) == 0)

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>