py3: check for None before comparing with integers

Comparing None and integers on Python 3 is not allowed and raise error.

Differential Revision: https://phab.mercurial-scm.org/D3614

mercurial/revlog.py

@@ -846,7 +846,7 b' class revlog(object):'
     def rawsize(self, rev):
         """return the length of the uncompressed text for a given revision"""
         l = self.index[rev][2]
-        if l >= 0:
+        if l is not None and l >= 0:
             return l
 
         t = self.revision(rev, raw=True)