send-email: fix mutt regex for grouped aliases

For example:
alias -group friends foo Foo Bar <foo@bar.com>

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Acked(-and-tested)-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Jeff King <peff@peff.net>

git-send-email.perl

@@ -401,7 +401,7 @@ sub split_addrs {
 my %parse_alias = (
 	# multiline formats can be supported in the future
 	mutt => sub { my $fh = shift; while (<$fh>) {
-		if (/^\s*alias\s+(\S+)\s+(.*)$/) {
+		if (/^\s*alias\s+(?:-group\s+\S+\s+)*(\S+)\s+(.*)$/) {
 			my ($alias, $addr) = ($1, $2);
 			$addr =~ s/#.*$//; # mutt allows # comments
 			 # commas delimit multiple addresses