Unstable release upgrades

[rarkins]

Default behaviour should definitely be that we don't update from stable to unstable. Any project wishing to try an unstable package release should do that manually.

However, if a project is already using an unstable release, they would presumably want access to upgrades, such as from release candidate 1 to release candidate 2 or from a release candidate to final release.

Hence it is a nice-to-have feature that any existing unstable versions are checked for possible upgrades.