revlog: fix pure nodemap to not access missing index entry

This bug was revealed by a3dacabd476b and a1f934573c0b.

mercurial/revlog.py

@@ -1107,7 +1107,9 class revlog(object):
 
     @util.propertycache
     def nodemap(self):
-        self.rev(self.node(0))
+        if self.index:
+            # populate mapping down to the initial node
+            self.rev(self.node(0))
         return self._nodecache
 
     def hasnode(self, node):