Limit realtime update by the number of line

When I edited a large file, it took much time to redraw signs. I wanted to disable realtime update when I edited a large file.
So I added `g:gitgutter_realtime_line_limit`. If the number of the file is more than this variable, realtime update is disabled. Because the default value of `g:gitgutter_realtime_line_limit` is `0`, this variable doesn't make a change as default.