Before, when `flatten`ing, the struct you flattened in could set the
help heading for all the following arguments. Now, we scope each
`flatten` to not do that.
I had originally intended to bake this into the initial / final methods
but that would have required some re-work to allow capturing state
between them that seemed unnecessary.
Fixes#2803
We normally set all app attributes at the end. This can be changed but
will require some work to ensure
- Top-level item's doc cmment ins our over flattened
- We still support `Args` / `Subcommand` be used to initialize an `App` when
creating a subcommand
In the mean time, this special cases `help_heading` to happen first.
We'll need this special casing anyways to address #2803 since we'll need
to capture the old help heading before addings args and then restore it
after. I guess we could unconditionally do that but its extra work /
boilerplate for when people have to dig into their what the derives do.
Fixes#2785
