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