git-send-email: allow leading white space on mutt aliases

mutt version 1.5.14 (perhaps earlier versions too) permits alias files to have
white space before the 'alias' keyword.

Signed-off-by: Michael Hendricks <michael@ndrix.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-send-email.perl

@@ -212,7 +212,7 @@ sub format_2822_time {
 my %parse_alias = (
 	# multiline formats can be supported in the future
 	mutt => sub { my $fh = shift; while (<$fh>) {
-		if (/^alias\s+(\S+)\s+(.*)$/) {
+		if (/^\s*alias\s+(\S+)\s+(.*)$/) {
 			my ($alias, $addr) = ($1, $2);
 			$addr =~ s/#.*$//; # mutt allows # comments
 			 # commas delimit multiple addresses