send-email: allow multiple emails using --cc, --to and --bcc

send-email: allow multiple emails using --cc, --to and --bcc

Accept a list of emails separated by commas in flags --cc, --to and
--bcc.  Multiple addresses can already be given by using these options
multiple times, but it is more convenient to allow cutting-and-pasting
a list of addresses from the header of an existing e-mail message,
which already lists them as comma-separated list, as a value to a
single parameter.

The following format can now be used:

    $ git send-email --to='Jane <jdoe@example.com>, mike@example.com'

Remove the limitation imposed by 79ee555b (Check and document the
options to prevent mistakes, 2006-06-21) which rejected every argument
with comma in --cc, --to and --bcc.

Signed-off-by: Mathieu Lienard--Mayor <Mathieu.Lienard--Mayor@ensimag.imag.fr>
Signed-off-by: Jorge Juan Garcia Garcia <Jorge-Juan.Garcia-Garcia@ensimag.imag.fr>
Signed-off-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
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>