Restore value of 'cpoptions'

Hi Andrew,

I accidentally noticed that output of
```vim
:set cpoptions
```
is
```
  cpoptions=aABceFs
```
while it should be
```
  cpoptions=aABceFs$
```
because I have this line in my `vimrc`:
```vim
set cpoptions+=$
```
After running `grep` I saw that `linediff.vim` saves old value of the `'cpoptions'` at the beginning of the script, then resets `'cpoptions'` to defaults, but **doesn't** restore it at the end. This very small pull request fixes the issue.

Cheers,
xaizek