explore history more thoroughly;

--- a/src/Pure/Admin/isabelle_cronjob.scala	Sat Nov 04 17:19:22 2023 +0100
+++ b/src/Pure/Admin/isabelle_cronjob.scala	Sat Nov 04 20:10:17 2023 +0100
@@ -212,7 +212,8 @@
 
         pick_recent(options.int("build_log_history") max history, 2) orElse
         pick_recent(200, 5) orElse
-        pick_recent(2000, 1)
+        pick_recent(2000, 50) orElse
+        pick_recent(0, 1)
       }
     }