When I had to pick a backup system, I considered Bup until I saw that there were no way to prune old backup ( https://github.com/bup/bup/blob/master/README.md#things-that... ).

This is really a stopblocker for me.

Obnam ( http://obnam.org ) is a similar tool but support forgetting old generations. However, it still suffers for youth problems and tends to corrupt backup repository when pruning old data on a remote server.

I'm really looking forward for a mature and feature full backup system based on git principes. Bup or obnam might be one of those.