template: better prune support in obsfate
successorssets don't returns good results for pruned commit, add a workaround
for simple cases.
A proper fix would require a large rework of successorssets algorithm, I will
send a separate series for this refactoring.
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
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273ce12ad8f1` will report some style violations on a very old commit).