Allow trees to trim to the top Nodes by NodeInfo

In a graph, NodeInfo maps one to one to the nodes, so it suffices to
just find the top NodeInfo s and only keep those nodes in the graph. In
a tree however, a single NodeInfo may map to many nodes. As of this
commit, a call to 'web 10' in pprof on a tree will return all the nodes
corresponding with the top 10 NodeInfo s.

internal/graph/graph.go

 	kept := o.KeptNodes
 	keepBinary := o.ObjNames
 	parentNodeMap := make(map[*Node]NodeMap, len(prof.Sample))
-nextSample:
 	for _, sample := range prof.Sample {
 		weight := o.SampleValue(sample.Value)
 		if weight == 0 {
 				}
 				n := nodeMap.findOrInsertLine(l, lines[lidx], keepBinary, kept)
 				if n == nil {
-					continue nextSample
+					continue
 				}
 				n.addSample(weight, labels, sample.NumLabel, o.FormatTag, false)
 				if parent != nil {

internal/report/report.go

 	nodeCutoff := abs64(int64(float64(totalValue) * o.NodeFraction))
 	edgeCutoff := abs64(int64(float64(totalValue) * o.EdgeFraction))
 
-	// Visual mode optimization only supports graph output, not tree.
 	// Do not apply edge cutoff to preserve tree structure.
 	if o.CallTree {
-		visualMode = false
 		if o.OutputFormat == Dot {
+			fmt.Println("WARNING: Trimming trees is unsupported.")
+			fmt.Printf("Tree will contain at least %d nodes\n", o.NodeCount)
 			cumSort = true
 		}
 		edgeCutoff = 0