[Borgbackup] Wish: option, that don't create an archive, if nothing has changed

[Hans-Peter Jansen]

Hi,

I've successfully moved from rdiff-backup to borg with great success. One 
feature of rdiff-backup, I like very much: don't create an archive, if nothing 
has changed. Of course, this has to be an dedicated create option.

Why: I'm backung up a lot of VMs, that only used occasionally. It's nice to 
see the real repo state, that is the last time, a machine from that repo has 
run.

What do you think?

Pete