[commands] Add Command.qualified_name to get the full command name.

This also sets `__str__` to do the same thing.

discord/ext/commands/formatter.py

@@ -117,32 +117,20 @@ class HelpFormatter:
         # odd one.
         return self.context.prefix.replace(user.mention, '@' + user.name)
 
-    def get_qualified_command_name(self):
-        """Retrieves the fully qualified command name, i.e. the base command name
-        required to execute it. This does not contain the command name itself.
-        """
-        entries = []
-        command = self.command
-        while command.parent is not None:
-            command = command.parent
-            entries.append(command.name)
-
-        return ' '.join(reversed(entries))
-
     def get_command_signature(self):
         """Retrieves the signature portion of the help page."""
         result = []
         prefix = self.clean_prefix
-        qualified = self.get_qualified_command_name()
         cmd = self.command
+        parent = cmd.full_parent_name
         if len(cmd.aliases) > 0:
             aliases = '|'.join(cmd.aliases)
             fmt = '{0}[{1.name}|{2}]'
-            if qualified:
+            if parent:
                 fmt = '{0}{3} [{1.name}|{2}]'
-            result.append(fmt.format(prefix, cmd, aliases, qualified))
+            result.append(fmt.format(prefix, cmd, aliases, parent))
         else:
-            name = prefix + cmd.name if not qualified else prefix + qualified + ' ' + cmd.name
+            name = prefix + cmd.name if not parent else prefix + parent + ' ' + cmd.name
             result.append(name)
 
         params = cmd.clean_params