git phlow deliver should fail if there is nothing to deliver

[JKrag]

Currently (v 3.1) git phlow does not check if you have made any commits, and will still try to deliver your branch. This is a cause for confusion and also takes time compared to the instant check of "up to date status".

Instead I suggest that deliver should check if you have any commits ahead of origin/ and present a nice warning like "The is nothing to deliver. Did you perhaps forget to run git wrapup first?"

I cannot at this time think of a scenario where we would need to be able to override this behaviour but please comment if you can think of a valid case - even obscure.