gn: Stop printing all help when the command is unknown

tools/gn/gn_main.cc

 // Copyright (c) 2013 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.
 
 #include "base/at_exit.h"
 #include "base/command_line.h"
 #include "base/strings/utf_string_conversions.h"
 #include "build/build_config.h"
 #include "tools/gn/commands.h"
 #include "tools/gn/err.h"
@@ skipping 55 matching lines @@
   }
 
   const commands::CommandInfoMap& command_map = commands::GetCommands();
   commands::CommandInfoMap::const_iterator found_command =
       command_map.find(command);
 
   int retval;
   if (found_command != command_map.end()) {
     retval = found_command->second.runner(args);
   } else {
-    Err(Location(),
-        "Command \"" + command + "\" unknown.").PrintToStdout();
-    commands::RunHelp(std::vector<std::string>());
+    Err(Location(), "Command \"" + command + "\" unknown.").PrintToStdout();
+    OutputString(
+        "Available commands (type \"gn help <command>\" for more details):\n");
+    for (const auto& cmd : commands::GetCommands())
+      PrintShortHelp(cmd.second.help_short);
+
     retval = 1;
   }
 
   exit(retval);  // Don't free memory, it can be really slow!
 }