Nested updates implemented have bugs around subcommands

[epage]

Discussed in #2591

^{Originally posted by epage July 14, 2021}
This was implemented in #1878 for #1837 based on #1836.

I've not dig into all of #1878's comments but at one point the thought was:

We don't know which fields the flattened struct consists of, so we delegate to the flattened type itself - it must know how to update itself.

This isn't always the case. From inspection

• SubCmd{ field: i32 } generates an augment update
• SubCmd(Args) is hard coded to call augment
• #[flatten] Subcommand(MoreCmds) is hard coded to call augment
• #[subcmmand] field: SubCmd calls augment update
• #[flatten] field: MoreArgs is hard coded to call augment

Example generated code from clap_derive/tests/flatten.rs where there is a mismatch in augment vs from

        fn augment_clap_for_update<'b>(app: clap::App<'b>) -> clap::App<'b> {
            {
                let app = app;
                let app = <Common as clap::IntoApp>::augment_clap(app);
                app
            }
        }
...
        fn update_from_arg_matches(&mut self, arg_matches: &clap::ArgMatches) {
            {
                #[allow(non_snake_case)]
                let common = &mut self.common;
                clap::FromArgMatches::update_from_arg_matches(common, arg_matches);
            }
        }
```</div>