more ambitious default as in former isatest;

--- a/src/Pure/Tools/build_history.scala	Tue Oct 04 14:26:05 2016 +0200
+++ b/src/Pure/Tools/build_history.scala	Tue Oct 04 14:32:44 2016 +0200
@@ -17,7 +17,7 @@
 
   private val default_rev = "tip"
   private val default_threads = 1
-  private val default_heap = 500
+  private val default_heap = 1000
   private val default_isabelle_identifier = "build_history"
 
   def build_history(