avoid conflict with generated settings of other_isabelle;

--- a/src/Pure/Admin/build_history.scala	Thu May 25 21:32:51 2017 +0200
+++ b/src/Pure/Admin/build_history.scala	Thu May 25 21:55:17 2017 +0200
@@ -405,7 +405,7 @@
     isabelle_repos_self: Path,
     isabelle_repos_other: Path,
     isabelle_repos_source: String = "http://isabelle.in.tum.de/repos/isabelle",
-    isabelle_identifier: String = default_isabelle_identifier,
+    isabelle_identifier: String = "remote_build_history",
     self_update: Boolean = false,
     push_isabelle_home: Boolean = false,
     progress: Progress = No_Progress,