GreaseMonkey (or TamperMonkey) script that adds a +1 button to GitHub Pull Requests. (Click "Raw" to install.)

PlusOne.user.js

// ==UserScript==
// @name       GitHub Pull Request Enhancements
// @namespace  http://tannermar.es
// @version    0.2
// @description  Add "Plus One" and "Retest" buttons to PR pages. Automatically add and remove "+1", "+2", and "NOT READY!" labels bast on comment contents. Included on all of github because pushState.
// @include      https://github.com/*
// @copyright  2014+, @tannermares
// ==/UserScript==

var emojiPlusOnes = function() {
  return jQuery(".comment-body:visible:has(.emoji[title=':+1:'])").length;
}

var rawPlusOnes = function() {
  return jQuery(".comment-body:visible:contains('+1')").length;
}

var plusOnesReceived = function() {
  return emojiPlusOnes() + rawPlusOnes();
}

var isPullRequestPage = function() {
  return !!window.location.href.match(/pull\/\d/)
}

var isButtonAlreadyPresent = function() {
  return jQuery(".plus-one-button").length > 0;
}

var setupButtons = function() {
  if (isButtonAlreadyPresent() || !isPullRequestPage()) {
    return;
  }

  //$("<button type='button' class='btn btn-sm retest-button'><span class='octicon octicon-sync'></span> Retest</button>")
  //.prependTo($('.gh-header-actions'))
  //.click(function(e) {
  //  setTimeout(function(){
  //    console.log('adding comment');
  //    $('#new_comment_field').val('retest').parents('.js-new-comment-form').find('.primary[type="submit"]').click();
  //  }, 50);
  //  setTimeout(function(){
  //     console.log('removing comment');
  //    $('div[id^=issuecomment]').last().find('.delete-button').click()
  //  }, 1000);
  //});

  $("<button type='button' class='btn btn-sm plus-one-button'><span class='octicon octicon-plus'></span> Plus one</button>")
  .prependTo($('.gh-header-actions'))
  .click(function(e) {
    setTimeout(function(){
      $('#new_comment_field').val(':+1:').parents('.js-new-comment-form').find('.btn-primary[type="submit"]').click();
    });
  });
  
  $("<button type='button' class='btn plus-one-button'><span class='octicon octicon-plus'></span> Plus one</button>")
  .appendTo($('#partial-new-comment-form-actions'))
  .click(function(e) {
    setTimeout(function(){
      $('#new_comment_field').val(':+1:').parents('.js-new-comment-form').find('.btn-primary[type="submit"]').click();
    });
  });
}

jQuery(function(){
  jQuery(document).on('ajaxComplete', function(){
    window.setTimeout(setupButtons, 50);
  });
});

👍