abandon: add support for --quiet

Also fix the normal output to write to stdout.

Change-Id: I6c117eea9cec08f5be9a44b90dbe9bf1f824ec95
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256114
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>