Use the code font for comment boxes and disable "Update branch" button.

GitHub fiddling.user.js

// ==UserScript==
// @name         GitHub fiddling
// @namespace    http://sonalytic.com
// @version      0.1
// @author       Till Hoffmann
// @match        https://github.com/*
// @grant        GM_addStyle
// @require      https://code.jquery.com/jquery-3.1.0.min.js
// ==/UserScript==

(function() {
    'use strict';
    jQuery.noConflict();
    GM_addStyle('.comment-form-textarea {font-family: Consolas, "Liberation Mono", Menlo, Courier, monospace}');
    jQuery("button:contains('Update branch')").hide();
    
    // TODO: add tab => 4 spaces script in the comment boxes
})();