shelve: move unshelve-finishing logic to a separate function
Finishing unshelve involves two steps now:
- stripping a changelog
- aborting a transaction
Obs-based shelve will not require these things, so isolating this logic
into a separate function where the normal/obs-shelve branching is
going to be implemented seems to be like a nice idea.
Behavior-wise this change moves 'unshelvecleanup' from being between
changelog stripping and transaction abortion to being after them.
I don't think this has any negative effects.
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