Bug 1637845 - Allow ./mach command --help to print out the help for both sub-commands and command arguments r=ahal

It is possible to have both a default command (with positional arguments) and
sub-commands (with arguments) in mach. If the subcommand exists, it
is dispatched to; if it doesn't the default command is called the positional
argument filled in.

However, when you run ./mach command --help, it will detect the subcommands
and only print out their help section.  If the default command has arguments,
they were not printed out.  Now they are.

Small papercuts in this patch are that the Default Command Arguments are
printed after the subcommands and that subcommand help without default
arguments have an extra newline after them. Both of these seem small
enough that the refactoring necessary to abate them is undesirable.

Differential Revision: https://phabricator.services.mozilla.com/D76505

python/mach/mach/dispatcher.py

@@ -313,16 +313,7 @@ class CommandAction(argparse.Action):
                 group = extra_groups[group_name]
             group.add_argument(*arg[0], **arg[1])
 
-    def _handle_command_help(self, parser, command, args):
-        handler = self._mach_registrar.command_handlers.get(command)
-
-        if not handler:
-            raise UnknownCommandError(command, 'query')
-
-        if handler.subcommand_handlers:
-            self._handle_subcommand_help(parser, handler, args)
-            return
-
+    def _get_command_arguments_help(self, handler):
         # This code is worth explaining. Because we are doing funky things with
         # argument registration to allow the same option in both global and
         # command arguments, we can't simply put all arguments on the same
@@ -362,6 +353,20 @@ class CommandAction(argparse.Action):
 
         self._populate_command_group(c_parser, handler, group)
 
+        return c_parser
+
+    def _handle_command_help(self, parser, command, args):
+        handler = self._mach_registrar.command_handlers.get(command)
+
+        if not handler:
+            raise UnknownCommandError(command, 'query')
+
+        if handler.subcommand_handlers:
+            self._handle_subcommand_help(parser, handler, args)
+            return
+
+        c_parser = self._get_command_arguments_help(handler)
+
         # Set the long help of the command to the docstring (if present) or
         # the command decorator description argument (if present).
         if handler.docstring:
@@ -398,9 +403,13 @@ class CommandAction(argparse.Action):
         if handler.docstring:
             parser.description = format_docstring(handler.docstring)
 
+        c_parser = self._get_command_arguments_help(handler)
+
         parser.formatter_class = argparse.RawDescriptionHelpFormatter
 
         parser.print_help()
+        print('')
+        c_parser.print_help()
 
     def _handle_subcommand_help(self, parser, handler, args):
         subcommand = set(args).intersection(list(handler.subcommand_handlers.keys()))