Ticket #44517: allow-setting-username-on-url-for-login.php

Index: src/wp-login.php
===================================================================
--- src/wp-login.php    (revision 43389)
+++ src/wp-login.php    (working copy)
@@ -1050,6 +1050,8 @@
 
                if ( isset( $_POST['log'] ) ) {
                        $user_login = ( 'incorrect_password' == $errors->get_error_code() || 'empty_password' == $errors->get_error_code() ) ? esc_attr( wp_unslash( $_POST['log'] ) ) : '';
+               } elseif ( isset( $_GET['username'] ) ) {
+                       $user_login = esc_attr( wp_unslash( $_GET['username'] ) );
                }
                $rememberme = ! empty( $_POST['rememberme'] );