revlog: don't include trailing nullrev in revlog.revs(stop=len(revlog))

This was an odd side effect of the nullid entry that's in the
index. The existing callers (mostly repair.py) seem to have handled it
fine. It doesn't seem intentional, and it's pretty surprising, so
let's remove that surprise.

Differential Revision: https://phab.mercurial-scm.org/D4015

mercurial/revlog.py

@@ -1070,12 +1070,15 b' class revlog(object):'
     def revs(self, start=0, stop=None):
         """iterate over all rev in this revlog (from start to stop)"""
         step = 1
+        length = len(self)
         if stop is not None:
             if start > stop:
                 step = -1
             stop += step
+            if stop > length:
+                stop = length
         else:
-            stop = len(self)
+            stop = length
         return xrange(start, stop, step)
 
     @util.propertycache