cli: optional message for issue comments

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.

crates/radicle-cli/src/commands/issue/args.rs

@@ -310,7 +310,7 @@ pub(crate) struct CommentArgs {
     /// The body of the comment
     #[arg(long, short)]
     #[arg(value_name = "MESSAGE")]
-    message: Message,
+    message: Option<Message>,
 
     /// Optionally, the comment to reply to. If not specified, the comment
     /// will be in reply to the issue itself
@@ -441,6 +441,7 @@ impl From<CommentArgs> for CommentAction {
             edit,
         }: CommentArgs,
     ) -> Self {
+        let message = message.unwrap_or(Message::Edit);
         match (reply_to, edit) {
             (Some(_), Some(_)) => {
                 unreachable!("the argument '--reply-to' cannot be used with '--edit'")