directed_category need only be convertible to one of directed_tag or undirected_tag

[SVN r34133]

libs/graph

@@ -1 +1 @@
-Subproject commit ebb669793646073a31f48c3512a1e17fcf094183
+Subproject commit e5848fee442221c5e309f66fa4f1b1fcb92e739a