How to submit a form in scrapy?

Your problem is that FormRequest.from_response() uses a different form - a "search form". But, you wanted it to use a "log in form" instead. Provide a formnumber argument:

yield FormRequest.from_response(response,
                                formnumber=1,
                                formdata=formdata,
                                clickdata={'name': 'commit'},
                                callback=self.parse1)

Here is what I see opened in the browser after applying the change (used "fake" user):

Solution using webdriver.

from selenium import webdriver
from selenium.webdriver.common.action_chains import ActionChains
import time
from scrapy.contrib.spiders import CrawlSpider

class GitSpider(CrawlSpider):

    name = "gitscrape"
    allowed_domains = ["github.com"]
    start_urls = ["https://www.github.com/login"]

    def __init__(self):
        self.driver = webdriver.Firefox()

    def parse(self, response):
        self.driver.get(response.url)
        login_form = self.driver.find_element_by_name('login')
        password_form = self.driver.find_element_by_name('password')
        commit = self.driver.find_element_by_name('commit')
        login_form.send_keys("yourlogin")
        password_form.send_keys("yourpassword")
        actions = ActionChains(self.driver)
        actions.click(commit)
        actions.perform()
        # by this point you are logged to github and have access 
        #to all data in the main menù
        time.sleep(3)
        self.driver.close()