Do not attempt commit comments for overly-large pushes

On Drupal.org we’ve set the queueing that surrounds commit commenting to time out after 20 minutes. With #3052333: Do not re-run successful or in-progress reposync queue items, these start executing again in another process, and once two of them are going, that blocks the queue.

These comments are generally not accomplishing much, like pushing all of core’s history to a sandbox, which will generate zero comments. So we can just skip processing if the commits are too numerous.