add_edge() now returns a valid edge descriptor whether or not the

edge was added


[SVN r8577]

libs/graph

@@ -1 +1 @@
-Subproject commit df9b4add99e226dae9100a5e239ce143f7423bc3
+Subproject commit 79864c4de29f27191d7ed33c24c91613f33424d6