gitweb: Serve text/* 'blob_plain' as text/plain with $prevent_xss

One of mechanism enabled by setting $prevent_xss to true is 'blob_plain'
view protection.  With XSS prevention on, blobs of all types except a
few known safe ones are served with "Content-Disposition: attachment" to
make sure they don't run in our security domain.

Instead of serving text/* type files, except text/plain (and including
text/html), as attachements, downgrade it to text/plain.  This way HTML
pages in 'blob_plain' (raw) view would be displayed in browser, but
safely as a source, and not asked to be saved.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/gitweb.perl

@@ -4752,7 +4752,15 @@ sub git_blob_plain {
 	# want to be sure not to break that by serving the image as an
 	# attachment (though Firefox 3 doesn't seem to care).
 	my $sandbox = $prevent_xss &&
-		$type !~ m!^(?:text/plain|image/(?:gif|png|jpeg))(?:[ ;]|$)!;
+		$type !~ m!^(?:text/[a-z]+|image/(?:gif|png|jpeg))(?:[ ;]|$)!;
+
+	# serve text/* as text/plain
+	if ($prevent_xss &&
+	    $type =~ m!^text/[a-z]+\b(.*)$!) {
+		my $rest = $1;
+		$rest = defined $rest ? $rest : '';
+		$type = "text/plain$rest";
+	}
 
 	print $cgi->header(
 		-type => $type,