shelve: move rebasing logic to a separate function
Rebasing restored shelved commit onto the right destination is done
differently in traditional and obs-based unshelve:
- for traditional, we just rebase it
- for obs-based, we need to check whether a successor of
the restored commit already exists in the destination (this
might happen when unshelving twice on the same destination)
This is the reason why this piece of logic should be in its own
function: to not have excessive complexity in the main function.