Emit `__GO_LOG_COMMAND_DEPTH` in `@go.log_command` output

[mbland]

Thinking about the upcoming JSON support and how to handle @go.log_command output, I'm thinking of emitting each line as something like:

{"d":_GO_LOG_COMMAND_DEPTH,"o":"line of output"}

It may also be useful to add the depth to the other @go.log calls as well.