Blender Git Loki

Add a 'comment' field to listdir revisions.

Also, make clear tags are only informative here.

Reason is, if the project is a success, we have to think big and
imagine asset engines managing catalogs of tens or hundreds of thousands
of entries (web ones e.g.), we cannot hold list of those in filebrowser!
So most of filetring will be 'deffered' to asset-engine.

We'll also probably have to implement a 'chunck listing' behavior
(to only list at most n entries in a row), for same reasons.

This will make things slightly more complicated :|