repo/init/sync: rework default git download output

When we download git sources, we get a progress bar (good) and we get
a dump of all the refs we downloaded (bad) as it can easily be 100+ if
not 1000+ depending on the project (for each git repo!).  Lets rework
the output behavior so that:
* quiet: Only errors.
* default: Progress bars (if on a tty).
* verbose: Full output (progress bars & downloaded refs).

Bug: https://crbug.com/gerrit/11293
Change-Id: I87a380075e79de6805f91095876dd1b37d32873a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256456


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Michael Mortensen <mmortensen@google.com>