send-email: suppress meaningless whitespaces in from field

send-email: suppress meaningless whitespaces in from field

Remove leading and trailing whitespaces in from field before
interepreting it to improve consistency with other options.  The
split_addrs function already take care of trailing and leading
whitespaces for to, cc and bcc fields.
The from option now:

 - has the same behavior when passing arguments like
   "  jdoe@example.com ", "\t jdoe@example.com " or
   "jdoe@example.com".

 - interprets aliases in string containing leading and trailing
   whitespaces such as " alias" or "alias\t" like other options.

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>