code: func (s *GitScanner) ScanPreviousVersions(ref string, since time.Time, cb GitScannerFoundPointer) error {
  	callback, err := firstGitScannerCallback(cb, s.FoundPointer)
  	if err != nil {
  		return err
  	}
  	return logPreviousSHAs(callback, ref, since)
  }
docs: // ScanPreviousVersions scans changes reachable from ref (commit) back to since.
  // Returns channel of pointers for *previous* versions that overlap that time.
  // Does not include pointers which were still in use at ref (use ScanRefsToChan
  // for that)