I’ve often thought this would make a good feature, though I’ve never seen a plugin for it. I think the term “comment” is probably throwing off your search. I wouldn’t think that this would be stored as a comment internally, but rather as metadata for the post. I did a search for plugins using the term “revision description” instead of revision comment, and I found one plugin:
https://wordpress.org/plugins/post-revision/
It displays the revision description on the front-end, so it isn’t exactly what you were looking for. Also, it only keeps track of the description for the latest revision, not older revisions. However, it might still offer some inspiration for someone looking to create a plugin like this.
It does store the revision comment as post metadata, just like I thought. However, there is a problem with this, and that is that post metadata isn’t versioned with revisions—which is why it only can keep track of the description for the last revision. But you might be able to use the WP Post Meta Revisions plugin to version that particular piece of metadata.
(I guess maybe this isn’t a real answer, but it is too long for a comment, and I thought that I should post it for posterity. 🙂