Fix 'path' option being replaced with runtimepath

We want to append to the existing 'path', instead of replacing it, because it might contain entries previously added by the user.

In my scenario, instead of a fuzzy finder, I have in my .vimrc

```vim
set path+=**
```

So, when I'm working on a Vim plugin, then my path (including `**`) gets replaced by the runtimepath, and it could just be appended instead.