more explicit pull;

--- a/Admin/cronjob/self_update	Sun Oct 30 13:15:14 2016 +0100
+++ b/Admin/cronjob/self_update	Mon Oct 31 14:21:23 2016 +0100
@@ -10,5 +10,5 @@
 cd "$HOME/cronjob"
 mkdir -p run log
 
-hg -R isabelle pull -q || echo "self_update pull failed"
+hg -R isabelle pull "http://isabelle.in.tum.de/repos/isabelle" -q || echo "self_update pull failed"
 hg -R isabelle update -C -q || echo "self_update update failed"