Otherwise it can make the log overwhelming when the number of rolls is higher
than the number of meaningful commits.

R=iannucci@chromium.org
BUG=620765

Change-Id: Iec0b41304d50091498d43abc4883e522b8cc9a2c
Reviewed-on: https://chromium-review.googlesource.com/448676
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>