[skip ci] add git pull

Author: Luke Lin

leetcode_generate.py

@@ -598,13 +598,15 @@ def write_readme(self):
 
     def push_to_github(self):
         strdate = datetime.datetime.now().strftime('%Y-%m-%d')
+        cmd_git_pull = 'git pull'
         cmd_git_add = 'git add .'
         cmd_git_commit = 'git commit -m "[skip ci] update at {date}"'.format(
             date=strdate
         )
         cmd_git_push = 'git push -u origin {branch}'.format(
             branch=REPO_BRANCH
         )
+        os.system(cmd_git_pull)
         os.system(cmd_git_add)
         os.system(cmd_git_commit)
         os.system(cmd_git_push)