input: Write a function to implement a basic version control system in Python that keeps track of changes in the code. The function should be implemented as a class named VersionControl with methods commit and restore. The commit method should save the current version of the code, and the restore method should revert to a previous version. The code should be stored in a file named "log_file.txt".
  class VersionControl:
      def __init__(self, lines):
          self.lines = lines
          self.mark = 0
   
      def commit(self):
          self.mark += 1
          log_file = open("log_file.txt", "a+")
          log_file.write(str(self.mark) + ": " + str(self.lines))
          log_file.close()
   
      def restore(self):
          log_file = open("log_file.txt", "r")
          lines = log_file.readlines()
          to_mark = self.mark-1
   
          self.lines = lines[to_mark]
          log_file.close()
  
  assert VersionControl(["def greet():\n", "   return 'Hello, world!'"]).commit() == None  # should write to log_file.txt
  # Additional tests required for restore functionality
output: 
  class VersionControl:
      def __init__(self, lines):
          self.lines = lines
          self.mark = 0
   
      def commit(self):
          self.mark += 1
          with open("log_file.txt", "a+") as log_file:
              log_file.write(str(self.mark) + ": " + str(self.lines) + "\n")
   
      def restore(self):
          with open("log_file.txt", "r") as log_file:
              lines = log_file.readlines()
              to_mark = self.mark - 1
              if to_mark >= 0 and to_mark < len(lines):
                  self.lines = eval(lines[to_mark].split(": ", 1)[1])
  