sync: cleanup output when not doing GC

Do not use a progress bar when not doing GC, and restrict activity in
that case to only repairing preciousObject state.

This also includes additional cleanup based on review comments from
previous changes.

Change-Id: I48581c9d25da358bc7ae15f40e98d55bec142331
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/353514


Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>