proper self_update of components -- do assume shared file-system;

--- a/Admin/cronjob/self_update	Tue Mar 27 13:59:01 2018 +0200
+++ b/Admin/cronjob/self_update	Wed Mar 28 11:54:18 2018 +0200
@@ -12,3 +12,4 @@
 
 hg -R isabelle pull "https://isabelle.in.tum.de/repos/isabelle" -q || echo "self_update pull failed"
 hg -R isabelle update -C -q || echo "self_update update failed"
+isabelle/bin/isabelle components -a || echo "self_update components failed"