scan-tree: Support gitweb.description.

Use gitweb.description instead of description file to determine
description, if option is enabled.

cgit.h

@@ -199,6 +199,7 @@ struct cgit_config {
 	int embedded;
 	int enable_filter_overrides;
 	int enable_gitweb_owner;
+	int enable_gitweb_desc;
 	int enable_http_clone;
 	int enable_index_links;
 	int enable_commit_graph;