Add an output to get the snap revision

[merlijn-sebrechts]

I have an action that pushed a snap to candidate and then creates an issue that asks to test the version in candidate.

I want this issue to show the revision number of the snap I just pushed. Therefore, it would be very useful if the action has an output that states which revision just got pushed to the store.

[sed-i]

Would be very helpful to avoid potential races.