The `From<String> for Message` implementation has an interesting
interaction between `clap` and how `Message` can be used from the
command line.
Note that `FromStr` is not implemented for `Message` – which usually
what is to be expected for use with `clap`, but in fact, `clap` also
allows `From<String>`.
This would make `Message` be a required option when using `rad issue comment`.
It is not possible to use `default_value_t` because that required an
implementation of `Display`, which in this case we do not want to
implement. Trying to use `default_value = "Message::Edit"` also would
not work – it uses that as the text.
So, the solution is to mark it as optional, and default to
`Message::Edit` when it is not specified.
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