gh-contrib-compare.user.js

// ==UserScript==
// @name        GitHub Contributor Compare
// @description Adds a compare link to forked repo commit pages
// @match       http://github.com/*/commit/*
// @match       https://github.com/*/commit/*
// ==/UserScript==

var onLoad = function(callback) {
    var script = document.createElement("script");
    var src = "http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js";
    script.setAttribute("src", src);
    script.addEventListener('load', function() {
        var script = document.createElement("script");
        script.textContent = "(" + callback.toString() + ")();";
        document.body.appendChild(script);
    }, false);
    document.body.appendChild(script);
}

onLoad(function() {
    var source = $('.fork-flag a')[0];
    if (source) {
        source = source.innerHTML;
        var branchUrl = '/api/v2/json/repos/show/' + source + '/branches';
        $.getJSON(branchUrl, function(response) {
            var branches = [];
            for (branch in response.branches) {
                branches.push(branch);
            }

            sourceUser = source.split('/')[0];
            var makeUrl = function(branch) {
                var urlParts = window.location.href.split('/');
                var commit = urlParts.pop();
                urlParts.push(sourceUser + ':' + branch + '...' + commit);
                return urlParts.join('/').replace('commit', 'compare');
            };

            var navBar = $($('.subnav-bar ul')[0]);
            var link = $('<a></a>').attr('href', '#');
            link.html('Compare to Source Branch (' + branches.length + ')');
            link.addClass('dropdown');
            
            var navItem = $('<li></li>').append(link);

            var dropDown = $('<ul></ul>');
            for (var i = 0; i < branches.length; i++) {
                var branch = branches[i];
                var branchItem = $('<li></li>')
                var branchLink = $('<a></a>').html(branch);
                branchLink.attr('href', makeUrl(branch));
                branchItem.append(branchLink);
                dropDown.append(branchItem);
            }

            navItem.append(dropDown);
            navBar.append(navItem);
        });
    }
});