git-send-email: allow whitespace in addressee list

When interactively supplying addresses to send an email to with
send-email, whitespace after the separation comma (as in 'list, jc')
wasn't ignored.  This meant that resolving of the alias ' jc' would
fail, sending an email only to list. With this patch, the optional
trailing whitespace is ignored.

Signed-off-by: Pieter de Bie <pdebie@ai.rug.nl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -442,7 +442,7 @@ if (!@to) {
 	}
 
 	my $to = $_;
-	push @to, split /,/, $to;
+	push @to, split /,\s*/, $to;
 	$prompting++;
 }