github: State updates (e.g. for CI) should be notified on state changes

[aperezdc]

The status webhook event can make the bot quite chatty e.g. for projects which run their CI every time there is a push done to the repository. This can be avoided by notifying only when the state changes, but not if it remains the same. Also, the first time the event is received for a commit, the event is always notified.

[aperezdc]

An interesting idea to consider is reusing the keystore plugin to persist statuses to disk. If that is done, it might be interesting to record the last time a status update was received for each commit, to be able to garbage-collect old entries from the database — though it is not likely that it will be a big set of data anyway.