cmd/digraph: print usage on -h

Also switch usage printing to using stderr always, as is common for Go
tools.

Change-Id: I240da4344fe6b51a418c0d4b0b7d7e7d5614c1d7
Reviewed-on: https://go-review.googlesource.com/129776
Reviewed-by: Alan Donovan <adonovan@google.com>

cmd/digraph/digraph.go

@@ -89,11 +89,12 @@ Example usage:
 `
 
 func main() {
+	flag.Usage = func() { fmt.Fprintln(os.Stderr, Usage) }
 	flag.Parse()
 
 	args := flag.Args()
 	if len(args) == 0 {
-		fmt.Println(Usage)
+		fmt.Fprintln(os.Stderr, Usage)
 		return
 	}