Patch to add settings.local.php support. Note that this won't work as a patch atm, it's just for reference.

settings.local.patch

commit f1f7f92eff14a56fd3eec137b4d25f8eae8b139c
Author: Cameron Tod <cameron.tod@gmail.com>
Date:   Mon Feb 17 12:32:35 2014 +0000

    Added local settings include, to ease debugging of memory issue.
    
diff --git a/.gitignore b/.gitignore
--- a/.gitignore
+++ b/.gitignore
@@ -2,4 +2,5 @@
 docroot/files
 docroot/sites/*/files
 docroot/sites/*/private
+settings.local.php
 
diff --git a/docroot/sites/default/settings.php b/docroot/sites/default/settings.php
--- a/docroot/sites/default/settings.php
+++ b/docroot/sites/default/settings.php
$conf['cache_class_cache_form'] = 'DrupalDatabaseCache';
+
+/**
+ * Include a local settings file if it exists.
+ */
+$local_settings = dirname(__FILE__) . '/settings.local.php';
+if (file_exists($local_settings)) {
+  include $local_settings;
+}