revset: wider definition of ascending and descending for spanset

Before this patches, empty spanset were seen as neither ascending nor
descending. This is mathematically wrong and create some edges case. We put
`isascending` and `isdescending` back on track so we can use them to simplify
some of the spanset code.

Benchmarks show no performances harmed in the process.

mercurial/revset.py

@@ -2835,10 +2835,10 b' class spanset(_orderedsetmixin):'
         return self
 
     def isascending(self):
-        return self._start < self._end
+        return self._start <= self._end
 
     def isdescending(self):
-        return self._start > self._end
+        return self._start >= self._end
 
     def filter(self, l):
         if self._start <= self._end: