explore older history;

--- a/src/Pure/Admin/isabelle_cronjob.scala	Sun May 14 15:13:56 2017 +0200
+++ b/src/Pure/Admin/isabelle_cronjob.scala	Sun May 14 15:16:38 2017 +0200
@@ -156,8 +156,8 @@
         }
 
         pick_days(options.int("build_log_history") max history) orElse
-        pick_days(100) orElse
-        pick_days(1000)
+        pick_days(200) orElse
+        pick_days(2000)
       })
     }
   }