Bug 995847 - Add -h and --help support to mach. r=glandium

* |mach --help| prints the global help message as |mach help|.
* |mach command --help| prints the command help message as
  |mach help command|.

python/mach/mach/dispatcher.py

@@ -83,19 +83,27 @@ class CommandAction(argparse.Action):
         into that parser. We then merge the results with the main
         ArgumentParser.
         """
-        if not values:
-            raise NoCommandError()
-
+        if namespace.help:
+            # -h or --help is in the global arguments.
+            self._handle_main_help(parser)
+            sys.exit(0)
+        elif values:
             command = values[0].lower()
             args = values[1:]
 
             if command == 'help':
-            if len(args):
+                if args and args[0] not in ['-h', '--help']:
+                    # Make sure args[0] is indeed a command.
                     self._handle_subcommand_help(parser, args[0])
                 else:
                     self._handle_main_help(parser)
-
                 sys.exit(0)
+            elif '-h' in args or '--help' in args:
+                # -h or --help is in the command arguments.
+                self._handle_subcommand_help(parser, command)
+                sys.exit(0)
+        else:
+            raise NoCommandError()
 
         handler = self._mach_registrar.command_handlers.get(command)
 

python/mach/mach/main.py

@@ -567,6 +567,9 @@ To see more help for a specific command, run:
             action='store_true', default=False,
             help='Do not prefix log lines with times. By default, mach will '
                 'prefix each output line with the time since command start.')
+        global_group.add_argument('-h', '--help', dest='help',
+            action='store_true', default=False,
+            help='Show this help message.')
 
         for args, kwargs in self.global_arguments:
             global_group.add_argument(*args, **kwargs)