Consider outputing progress information, even when output is redirected.

It seems btrbk uses pv to indicate progress.
pv does not output progress when the output is redirected. This means that when btrbk is used in an automated fashion, e.g. with systemd timers, the log will not show progress.
This might make sense in some use cases, but for long running backups it can be a problem, especially as btrbk can stall when network problems are encountered, leaving the user to guess whether btrbk has stalled, or is running a long backup.
Maybe an option could be added to use pv's -n flag? This gives _a lot_ more output, but would allow users to monitor btrbk progress in logs.
As a bonus; pv's output could be filtered, to give less output than the -n option does.

Start by locating where btrbk invokes pv and how redirected output is handled. Compare pv's -n behavior with the current logging path, then determine an option and filtering approach that provides useful progress in automated logs without excessive output.

Written by the indexing model from the issue text.