Bug 1139100 - Apply proper checking for command existence; r=ahal

Previously, we raised errors attempting to register a mach sub-command
if the parent command was not defined on the Registrar. This broke B2G
because its mach command Registrar imposes restrictions on which
commands it exposes. Commands there were not getting registered on the
Registrar, leading mach to give a false positive that the parent command
was never defined.

We change the verification logic to take present but unregistered
commands into consideration and to skip registering sub-commands if the
parent isn't present in the Registrar.

python/mach/mach/decorators.py

@@ -47,6 +47,8 @@ def CommandProvider(cls):
         if len(spec.args) == 2:
             pass_context = True
 
+    seen_commands = set()
+
     # We scan __dict__ because we only care about the classes own attributes,
     # not inherited ones. If we did inherited attributes, we could potentially
     # define commands multiple times. We also sort keys so commands defined in
@@ -63,6 +65,8 @@ def CommandProvider(cls):
         if command_name is None:
             continue
 
+        seen_commands.add(command_name)
+
         if conditions is None and Registrar.require_conditions:
             continue
 
@@ -106,10 +110,13 @@ def CommandProvider(cls):
         if not command:
             continue
 
-        if command not in Registrar.command_handlers:
+        if command not in seen_commands:
             raise MachError('Command referenced by sub-command does not '
                 'exist: %s' % command)
 
+        if command not in Registrar.command_handlers:
+            continue
+
         arguments = getattr(value, '_mach_command_args', None)
         argument_group_names = getattr(value, '_mach_command_arg_group_names', None)