more ambitious default;

--- a/src/Pure/Admin/build_history.scala	Fri Oct 21 16:46:02 2016 +0200
+++ b/src/Pure/Admin/build_history.scala	Fri Oct 21 17:24:43 2016 +0200
@@ -97,7 +97,7 @@
 
   private val default_rev = "tip"
   private val default_multicore = (1, 1)
-  private val default_heap = 1000
+  private val default_heap = 1500
   private val default_isabelle_identifier = "build_history"
 
   def build_history(