find List ID if not passed

import-mbox doesn't seem to pass List ID unless
specified via CLI. Make compute_updates determine
it if not set.
diff --git a/tools/archiver.py b/tools/archiver.py
index b3e87d8..d8efc5f 100644
--- a/tools/archiver.py
+++ b/tools/archiver.py
@@ -232,7 +232,8 @@
         """
 
         ojson = None
-
+        if not lid:
+            lid= msg.get('list-id')
         if self.cropout:
             crops = self.cropout.split(" ")
             # Regex replace?