revlog: fix pure nodemap to not access missing index entry

This bug was revealed by a3dacabd476b and a1f934573c0b.

mercurial/revlog.py

@@ -1107,6 +1107,8 b' class revlog(object):'
 
     @util.propertycache
     def nodemap(self):
+        if self.index:
+            # populate mapping down to the initial node
         self.rev(self.node(0))
         return self._nodecache