Provide diffs stats for Rapid Diffs client consumption (new endpoint)

We need to provide diff stats from the server to the client through a new (separate) endpoint.

Response format should be:

{
  diff_stats: {
    added_lines: 1,
    removed_lines: 1,
    diffs_count: 1
  },
  overflow: { // null, in case we don't hit diff limits
    visible_count: 10, // needed for 'Showing <visible_count> out of <diffs_count> files'
    emailPath: '',
    diffPath: '',
  }
}

More params and response data could be added in subsequent work.