Add beginnings of new cr command

go/src/infra/tools/cr/help.go

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// The 'cr help' command. It is implemented here, rather than in the cmd/
+// package, because it needs to be able to reference the other subcommands.
+
+package main
+
+import (
+        "flag"
+        "fmt"
+
+        "infra/tools/cr/lib/subcommand"
+)
+
+var shortHelp = "Print help for a subcommand."
+
+var longHelp = `The help subcommand prints long-form help for the top-level cr command
+and its subcommands. Run 'cr help' for a list of available commands.
+
+Examples:
+  cr help             # print top-level help and list of commands
+  cr help firstrun    # print help for the 'firstrun' subcommand
+  cr firstrun --help  # same as above`
+
+func helpRun(flags *flag.FlagSet) error {
+        if flags.NArg() == 0 {
+                printCrHelp()
+                return nil
+        }
+        if cmdForHelp := subcommand.Get(flags.Arg(0)); cmdForHelp != nil {
+                cmdForHelp.InitFlags(flags)
+                cmdForHelp.Help(flags)
+                return nil
+        } else {
+                return fmt.Errorf("Unrecognized subcommand for help '%v'.\n", flags.Arg(0))
+        }
+}
+
+var helpCmd = subcommand.New("help", shortHelp, longHelp, nil, helpRun)