evade odd connection problems to https://isabelle.in.tum.de from some remote hosts;

--- a/Admin/cronjob/self_update	Wed Jul 18 16:44:01 2018 +0200
+++ b/Admin/cronjob/self_update	Wed Jul 18 17:01:12 2018 +0200
@@ -11,7 +11,7 @@
 mkdir -p run log
 
 {
-  hg -R isabelle pull "https://isabelle.in.tum.de/repos/isabelle" || echo "self_update pull failed" >&2
+  hg -R isabelle pull "https://isabelle.sketis.net/repos/isabelle" || echo "self_update pull failed" >&2
   hg -R isabelle update -C || echo "self_update update failed" >&2
   isabelle/bin/isabelle components -a 2>&1 || echo "self_update components failed" >&2
 } > run/self_update.out

--- a/src/Pure/Admin/isabelle_cronjob.scala	Wed Jul 18 16:44:01 2018 +0200
+++ b/src/Pure/Admin/isabelle_cronjob.scala	Wed Jul 18 17:01:12 2018 +0200
@@ -23,7 +23,7 @@
   val current_log = main_dir + Path.explode("run/main.log")  // owned by log service
   val cumulative_log = main_dir + Path.explode("log/main.log")  // owned by log service
 
-  val isabelle_repos_source = "https://isabelle.in.tum.de/repos/isabelle"
+  val isabelle_repos_source = "https://isabelle.sketis.net/repos/isabelle"
   val isabelle_repos = main_dir + Path.explode("isabelle")
   val afp_repos = main_dir + Path.explode("AFP")