unshelve: unify logic around creating an unshelve changeset
This is a follow-up patch to 5162753c4c14 on addressing reviews
on the commit. This unifies the logic around creating an unshelve
changeset.
Differential Revision: https://phab.mercurial-scm.org/D6683
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
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273ce12ad8f1` will report some style violations on a very old commit).