revlog: do inclusive descendant testing (API)

In many other places, a revision is considered a descendant of itself.  We
update the behavior of `revlog.descendant()` to match this. (for example.
`revlog.isancestor` does inclusive testing).

No tests break, so it seems safe to do so.

This will make it easier to use a more efficient implementation in a later
changeset.

mercurial/revlog.py

@@ -1378,6 +1378,8 b' class revlog(object):'
     def descendant(self, start, end):
         if start == nullrev:
             return True
+        elif start == end:
+            return True
         for i in self.descendants([start]):
             if i == end:
                 return True