imap.preformattedHTML to tell Thunderbird to send non-flowed text

imap.preformattedHTML to tell Thunderbird to send non-flowed text

Many e-mail based development communities require non-flowed text to carry
patches to prevent whitespaces from getting mangled, but there is no easy
way to tell Thunderbird MUA not to use format=flowed, unless you configure
it to do so unconditionally for all outgoing mails.

A workaround for users who use git-imap-send is to wrap the patch in "pre"
element in the draft folder as an HTML message, and tell Thunderbird to
send "text only".  Thunderbird turns such a message into a non-flowed
plain text when sending it out, which is what we want for patch e-mails.

Signed-off-by: Jeremy White <jwhite@codeweavers.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>