revlog: allow reverse iteration with revlog.revs

We often need to perform rev iteration in reverse order. This
changeset makes it possible to do so, in order to avoid costly reverse
or reversed() calls later.

mercurial/revlog.py

@@ -257,11 +257,14 b' class revlog(object):'
         return iter(xrange(len(self)))
     def revs(self, start=0, stop=None):
         """iterate over all rev in this revlog (from start to stop)"""
-        if stop is None:
-            stop = len(self)
+        step = 1
+        if stop is not None:
+            if start > stop:
+                step = -1
+            stop += step
         else:
-            stop += 1
-        return xrange(start, stop)
+            stop = len(self)
+        return xrange(start, stop, step)
 
     @util.propertycache
     def nodemap(self):