wireprotov2: establish a type for representing command response It will be desirable to have a higher-level type for representing command responses. This will allow us to do nicer things. For now, the instance encapsulates existing logic. It is still a bit primitive. But we're slowly making things better. Version 1 protocols have a wrapping layer that decodes the raw string data into a data structure and that data structure is sent to the future. Version 2 doesn't yet have this layer and the future is receiving the raw wire response. Hence why debugcommands needed to be taught about the response type. Differential Revision: https://phab.mercurial-scm.org/D3380