revlog: disallow setting uncompressed length to None

Backed out changeset a4942675de6b

mercurial/revlog.py

@@ -855,7 +855,7 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 is not None and l >= 0:
+        if l >= 0:
             return l
 
         t = self.revision(rev, raw=True)