staging doesn't deal well with forced pushes

we had some forced fun on the qtbase/containers branch. the result was that staging the next change was rejected with "requires recursive merge". once i made a forced push to staging as well, things were fine again.

so it would be good if the staging logic could detect rewinds of the target branch and rewind the statging branch, too (and on the way rebase all staged changes, which can cause them to be unstaged with a merge conflict). this should probably happen at the time of the forced pushing already, and latest when the integration starts (when no new staging attempts triggered the rebase already). consider the integration branch, too.