update graph: don't validate all paths of subgraphs

it's important that every node in the overall graph have some
path to the head, but not for a subgraph

pkg/updates/memory.go

@@ -98,7 +98,7 @@ func (m *MemorySource) Subgraph(head string) (Source, error) {
 		}
 	}
 
-	return newMemorySourceFromValidatedNodes(nodeSet, edges, orderedNodes)
+	return &MemorySource{Nodes: nodeSet, Edges: edges, OrderedNodes: orderedNodes}, nil
 }
 
 func (m *MemorySource) validateAllNodesPathToHead() error {