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>

git-send-email.perl

@@ -761,6 +761,7 @@ if (!$force) {
 }
 
 if (defined $sender) {
+	$sender =~ s/^\s+|\s+$//g;
 	($sender) = expand_aliases($sender);
 } else {
 	$sender = $repoauthor || $repocommitter || '';