This avoids the performance overhead of the function call every time `dirname( __FILE__ )` was used instead of `__DIR__`.
This commit also includes:
* Removing unnecessary parentheses from `include`/`require` statements. These are language constructs, not function calls.
* Replacing `include` statements for several files with `require_once`, for consistency:
* `wp-admin/admin-header.php`
* `wp-admin/admin-footer.php`
* `wp-includes/version.php`
Props ayeshrajans, desrosj, valentinbora, jrf, joostdevalk, netweb.
Fixes#48082.
Built from https://develop.svn.wordpress.org/trunk@47198
git-svn-id: http://core.svn.wordpress.org/trunk@46998 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Sergey Biryukov
Andrew Ozz