add config.hdf to large_memory test

This probably needs a repo to check repo contention

hphp/test/large_memory/config.hdf

@@ -0,0 +1,7 @@
+Repo {
+  Local.Mode = r-
+  Central.Path = /var/facebook/hhvm/cli.hhbc
+  Eval.Mode = readonly
+  Commit = true
+  DebugInfo = false
+}