revpair: simplify revpair by always relying on smartset.first/last
I thinkt the code was written the way it was because it comes from
97b2f26dfc43 (revpair: smartset compatibility, 2014-03-20) and the
first/last methods came only later, in
228b0aafb1ce (smartset: add
first and last methods, 2014-10-06).
Differential Revision: https://phab.mercurial-scm.org/D5687
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