Problem: The matchparen plugin is slow on a long line.
Solution: Don't use a regexp to get char at and before cursor.
(zeertzjq)
Example:
```vim
call setline(1, repeat(' foobar', 100000))
runtime plugin/matchparen.vim
normal! $hhhhhhhh
```
closes: vim/vim#1556881e7513c86
cf44121f7f
