`ostree commit --table-output` can already do this, and since we're
super noisy for `compose tree` right now, we might as well print this.
It's useful to see how many new objects were created.

Until we finally do the "store packages as ostree commits and union"
thing, doing commits can be slow, particularly into archive repos
where we pay lots of cost in gzip.

Let's show a progress bar.  The implementation here uses a background
thread which communicates with the "UI" via atomics.  The UI uses a
timer - and if stdout isn't a tty, we assume it's Jenkins or something
and dial updates back to every 5 seconds to avoid spamming output.