cmd/go: add tool -n flag

As in gdb $(go tool -n 6g).

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5689066

src/cmd/go/tool.go

@@ -17,12 +17,15 @@ import (
 
 var cmdTool = &Command{
 	Run:       runTool,
-	UsageLine: "tool command [args...]",
+	UsageLine: "tool [-n] command [args...]",
 	Short:     "run specified go tool",
 	Long: `
 Tool runs the go tool command identified by the arguments.
 With no arguments it prints the list of known tools.
 
+The -n flag causes tool to print the command that would be
+executed but not execute it.
+
 For more about each tool command, see 'go tool command -h'.
 `,
 }
@@ -32,8 +35,14 @@ var (
 	toolGOARCH    = runtime.GOARCH
 	toolIsWindows = toolGOOS == "windows"
 	toolDir       = build.ToolDir
+
+	toolN bool
 )
 
+func init() {
+	cmdTool.Flag.BoolVar(&toolN, "n", false, "")
+}
+
 const toolWindowsExtension = ".exe"
 
 func tool(name string) string {
@@ -67,6 +76,11 @@ func runTool(cmd *Command, args []string) {
 		setExitStatus(3)
 		return
 	}
+
+	if toolN {
+		fmt.Printf("%s %s\n", toolPath, strings.Join(args[1:], " "))
+		return
+	}
 	toolCmd := &exec.Cmd{
 		Path:   toolPath,
 		Args:   args,