Feature request: Force a new buildid on staging changes

[coolo]

We have one hole in the staging workflow: if unselecting packages that were unresolvable, the scheduler checks the new state and finds that nothing changed (outside of the build status) and doesn't trigger a new buildid in the repository and no event either.

This means we have no way to re-trigger a check job. So we need some way from the outside to force a new buildid on changes we do (the staging workflow could do this itself, but we need this for all projects following the status api).