bug#41409: “guix pull” should show a progress bar for channel updates

[Ricardo Wurmus]

When I run “guix pull” (or “guix time-machine”) I see this message

   Updating channel 'guix' from Git repository at 
'https://git.savannah.gnu.org/git/guix.git'...

followed by disconcerting silence.  I can’t tell if it’s doing
something, nor can I see what the progress is.

Would be nice to have a progress bar or at least a spinner.

-- 
Ricardo