Merge "installer: Add a defined check to overrideConfig method"

diff --combined includes/installer/Installer.php
index 33d4fcc,ffa01c7..f6a5c41
--- 1/includes/installer/Installer.php
--- 2/includes/installer/Installer.php
+++ b/includes/installer/Installer.php
@@@ -1764,7 -1764,9 +1764,9 @@@ abstract class Installer 
        public static function overrideConfig() {
                // Use PHP's built-in session handling, since MediaWiki's
                // SessionHandler can't work before we have an object cache set up.
-               define( 'MW_NO_SESSION_HANDLER', 1 );
+               if ( !defined( 'MW_NO_SESSION_HANDLER' ) ) {
+                       define( 'MW_NO_SESSION_HANDLER', 1 );
+               }
  
                // Don't access the database
                $GLOBALS['wgUseDatabaseMessages'] = false;
@@@ -1803,7 -1805,7 +1805,7 @@@
        /**
         * Add an installation step following the given step.
         *
 -       * @param callable $callback A valid installation callback array, in this form:
 +       * @param array $callback A valid installation callback array, in this form:
         *    [ 'name' => 'some-unique-name', 'callback' => [ $obj, 'function' ] ];
         * @param string $findStep The step to find. Omit to put the step at the beginning
         */