[vim] Try to make 'dir' option compatible with &autochdir

When 'dir' option is passed to fzf#run(), the current working directory
is temporarily changed to the given directory, and restored at the end.
However, this behavior is not compatible with &autochdir. This commit
introduces a heuristic to determine whether or not to restore the
previous working directory.

Related: https://github.com/junegunn/fzf.vim/issues/70