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

mercurial/scmutil.py

@@ -672,17 +672,8 b' def revpair(repo, revs):'
 
     l = revrange(repo, revs)
 
-    if not l:
-        first = second = None
-    elif l.isascending():
-        first = l.min()
-        second = l.max()
-    elif l.isdescending():
-        first = l.max()
-        second = l.min()
-    else:
-        first = l.first()
-        second = l.last()
+    first = l.first()
+    second = l.last()
 
     if first is None:
         raise error.Abort(_('empty revision range'))