suppress ISABELLE_ROOT after init, to avoid conflict with ISABELLE_HOME when folding file names in "isabelle jedit" command-line tool;

--- a/src/Pure/Tools/main.scala	Thu Feb 04 13:21:47 2016 +0100
+++ b/src/Pure/Tools/main.scala	Thu Feb 04 16:30:01 2016 +0100
@@ -105,6 +105,7 @@
         val env = env0.asInstanceOf[java.util.Map[String, String]]
         env.put("ISABELLE_HOME", File.platform_path(isabelle_home))
         env.put("ISABELLE_HOME_USER", File.platform_path(isabelle_home_user))
+        env.remove("ISABELLE_ROOT")
       }
     }