The --threaded option controls whether the In-Reply-To header will be set on
any emails sent. The current behavior is to always set this header, so this
option is most useful in its negated form, --no-threaded. This behavior can
also be controlled through the 'sendemail.threaded' config setting.
Signed-off-by: Adam Roben <aroben@apple.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>