smartset: change repr of spanset to show revisions as half-open range
Before, an empty spanset was displayed as '<spanset+ 0:-1>', which seemed
confusing.
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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).