log_checkpoint's "0 transaction log file(s) added" is extremely misleading

Hi,

While in theory correct, I think $subject is basically meaningless
because other backends may have added thousands of new segments. Yes, it
wasn't the checkpointer, but that's not particularly relevant
imo. Additionally, afaics, it will only ever be 0 or 1.

I think we should either remove that part of the log output, or make it
display the number of segments added since the beginning of the
checkpoint.

Greetings,

Andres Freund