graphviz: don't draw edges multiple times

From a patch from Ian Godin <iangodin@gmail.com>.

src/graphviz.cc

@@ -19,11 +19,11 @@
 #include "graph.h"
 
 void GraphViz::AddTarget(Node* node) {
-  if (visited_.find(node) != visited_.end())
+  if (visited_nodes_.find(node) != visited_nodes_.end())
     return;
 
   printf("\"%p\" [label=\"%s\"]\n", node, node->path().c_str());
-  visited_.insert(node);
+  visited_nodes_.insert(node);
 
   Edge* edge = node->in_edge();
 
@@ -33,6 +33,10 @@ void GraphViz::AddTarget(Node* node) {
     return;
   }
 
+  if (visited_edges_.find(edge) != visited_edges_.end())
+    return;
+  visited_edges_.insert(edge);
+
   if (edge->inputs_.size() == 1 && edge->outputs_.size() == 1) {
     // Can draw simply.
     // Note extra space before label text -- this is cosmetic and feels

src/graphviz.h

@@ -19,14 +19,16 @@
 using namespace std;
 
 struct Node;
+struct Edge;
 
 /// Runs the process of creating GraphViz .dot file output.
 struct GraphViz {
   void Start();
   void AddTarget(Node* node);
   void Finish();
 
-  set<Node*> visited_;
+  set<Node*> visited_nodes_;
+  set<Edge*> visited_edges_;
 };
 
 #endif  // NINJA_GRAPHVIZ_H_