send-email: visually distinguish sendmail aliases parser warnings

Although emitted to stderr, warnings from the sendmail aliases parser
are not visually distinguished as such, and thus can easily be
overlooked in the normal noisy send-email output.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -522,12 +522,12 @@ my %parse_alias = (
 
 		# warn on lines that contain quotes
 		elsif (/"/) {
-			print STDERR "sendmail alias with quotes is not supported: $_\n";
+			print STDERR "warning: sendmail alias with quotes is not supported: $_\n";
 		}
 
 		# warn on lines that continue
 		elsif (/^\s|\\$/) {
-			print STDERR "sendmail continuation line is not supported: $_\n";
+			print STDERR "warning: sendmail continuation line is not supported: $_\n";
 		}
 
 		# recognize lines that look like an alias
@@ -538,7 +538,7 @@ my %parse_alias = (
 
 		# warn on lines that are not recognized
 		else {
-			print STDERR "sendmail line is not recognized: $_\n";
+			print STDERR "warning: sendmail line is not recognized: $_\n";
 		}}},
 
 	gnus => sub { my $fh = shift; while (<$fh>) {