Set GC threshold high at start of initialisation

I did have this in here before and the code to set it back was still
at the end of the file, so not sure what happened there.  I guess I
accidentally removed it at some point.

config.org

@@ -2,6 +2,17 @@
 #+AUTHOR: Camden Dixie O'Brien
 #+ATTR_LATEX: :float t
 
+* Prelude
+  Before doing anything else, set a higher [[help:gc-cons-threshold][gc-cons-threshold]] (I'm
+  using 100 MiB here) so that we're not garbage collecting during
+  initialisation to make startup a little faster.  We'll save the
+  original value to restore it at the end of initialisation.
+
+  #+begin_src emacs-lisp
+    (setq original-gc-cons-threshold gc-cons-threshold)
+    (setq gc-cons-threshold (* 100 1024 1024))
+  #+end_src
+
 * Customize
   Hey, customize, leave my ~/.emacs.d/init.el alone!
 
@@ -1667,12 +1678,10 @@
      (setq god-exempt-predicates nil)
    #+end_src
 
-* Fin
-  Now that start-up is finished, [[help:gc-cons-threshold][gc-cons-threshold]] and
-  [[help:gc-cons-percentage][gc-cons-percentage]] need to be set back to reasonable values to avoid
-  memory usage getting too high.
+* Coda
+  Now that initialization is finished, [[help:gc-cons-threshold][gc-cons-threshold]] should be set
+  back to its default value:
 
   #+begin_src emacs-lisp
-    (setq gc-cons-threshold 1000000)
-    (setq gc-cons-percentage 0.2)
+    (setq gc-cons-threshold original-gc-cons-threshold)
   #+end_src