GN: Change gen command syntax, support relative dirs.

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 "tools/gn/commands.h"
 #include "tools/gn/err.h"
 #include "tools/gn/location.h"
@@ skipping 35 matching lines @@
 
   std::string command;
   if (cmdline.HasSwitch("help") || cmdline.HasSwitch("h")) {
     // Make "-h" and "--help" default to help command.
     command = commands::kHelp;
   } else if (cmdline.HasSwitch("version")) {
     // Make "--version" print the version and exit.
     OutputString(std::string(LAST_CHANGE) + "\n");
     exit(0);
   } else if (args.empty()) {
-    command = commands::kGen;
+    // No command, print error and exit.
+    Err(Location(), "No command specified.",
+        "Most commonly you want \"gn gen <out_dir>\" to make a build dir.\n"
+        "Or try \"gn help\" for more commands.").PrintToStdout();
+    return 1;
   } else {
     command = args[0];
     args.erase(args.begin());
   }
 
   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>());
     retval = 1;
   }
 
   exit(retval);  // Don't free memory, it can be really slow!
   return retval;
 }