format: do not group '--' arg

The group_arg_value option allow to put --key and its value on the
same line on multiline arguments. However, when the options is
`--`, it is an argument separator, and should not be grouped with
the following argument.

Author: bruchar1

mesonbuild/mformat.py

@@ -670,7 +670,7 @@ def visit_ArgumentNode(self, node: mparser.ArgumentNode) -> None:
             if self.config.group_arg_value:
                 for arg in node.arguments[:-1]:
                     group_args = False
-                    if isinstance(arg, mparser.StringNode) and arg.value.startswith('--'):
+                    if isinstance(arg, mparser.StringNode) and arg.value.startswith('--') and arg.value != '--':
                         next_arg = node.arguments[arg_index + 1]
                         if isinstance(next_arg, mparser.StringNode) and not next_arg.value.startswith('--'):
                             group_args = True