shelve: fix crash on unshelve without .shelve metadata file
Follow up for c67c94c0e7ae and 38373da1af02.
The inline comment says "we should keep track of the unshelve node in case
we need to reuse it." Perhaps such case isn't tested, and this patch does
NOT add a test for the reuse of the unbundled revision.
Also, I have no idea what should be done if new revision is unbundled
because of "node not in repo".
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