CLI should show progress for longer duration scans

Currently the cli outputs when the entire scan is complete. This is fine for smaller number of commits which complete within seconds, but for larger number of commits (10k+) the scan time is longer and so the cli appears stuck until it has completed.

The user should be notified with may be something like a percent completed like this:
```sh
$ disclosure scan
0%...10%...20%...50%...100.
# followed by the scan results
```

Start at the CLI scan entry point and trace how commits are processed until the final output is produced. Try a scan with more than 10k commits, then add progress reporting that reaches 100% before the scan results and verify that smaller scans still complete normally.

Written by the indexing model from the issue text.