have a default since for release prepare

[timdiggins]

when you're preparing a release it would be nice if you could just name the target release, and it would use the most recent tag as the default since release (release -s since target)

however it should really output what the default is, and ask for confirmation

$ release prepare 12345.1.1
  > assuming -s 12345.1.0
  > RETURN to confirm or ctrl-c (or anything) to abort