t9001-send-email: refactor header variable fields replacement

t9001-send-email: refactor header variable fields replacement

Create a function which replaces Date, Message-Id and
X-Mailer lines generated by git-send-email by a specific string:

Date:.*$       -> Date: DATE-STRING
Message-Id:.*$ -> Message-Id: MESSAGE-ID-STRING
X-Mailer:.*$   -> X-Mailer: X-MAILER-STRING
Signed-off-by: Remi Lespinet <remi.lespinet@ensimag.grenoble-inp.fr>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>