add Cmd::Repaint

[sujiacong]

I need to rely on this refresh-only action when implementing a list function similar to bash complete. Do you think this PR can be merged into the main branch?

[gwenn]

Repaint seems to be the norm:

• https://docs.rs/reedline/latest/reedline/enum.ReedlineEvent.html#variant.Repaint
• https://docs.rs/termwiz/latest/termwiz/lineedit/enum.Action.html#variant.Repaint
• https://github.com/AmokHuginnsson/replxx/blob/1f149bfe20bf6e49c1afd4154eaf0032c8c2fda2/include/replxx.hxx#L207

[gwenn]

Thanks.