revset: fix spanset.isascending() to honor sort() or reverse() request

Because spanset.isascending() ignored the ascending flag, the result of
"fullreposet() & x" was always sorted in ascending order.

The test case is carefully chosen to call fullreposet.__and__.

mercurial/revset.py

@@ -2913,10 +2913,10 b' class _spanset(abstractsmartset):'
             return abs(self._end - self._start) - count
 
     def isascending(self):
-        return self._start <= self._end
+        return self._ascending
 
     def isdescending(self):
-        return self._start >= self._end
+        return not self._ascending
 
     def first(self):
         if self._ascending:

tests/test-revset.t

@@ -496,6 +496,9 b' Test explicit numeric revision'
   2
   1
   0
+  $ log 'reverse(all()) & filelog(b)'
+  4
+  1
   $ log 'rev(5)'
   5
   $ log 'sort(limit(reverse(all()), 3))'