Fully support partial update

[renatav]

In some cases, it would make sense to only validate and update the authentication repository and some target repositories. However, the success of such an update should not result in updating last_validated_commit since we cannot know if there exists a problem with an excluded target repository. On the other had, it would be nice to skip validation of the already validated repositories the next time update is run, and only check those which were previously excluded. This additional information can be stored in the config directory and loaded when the next update is started.