jumplist: go to prev/next entry from another file

```
I often use the jumplist, and then want to advance in it backwards/forwards per 
file.

It would be helpful if e.g. `g<C-o>` and `g<C-i>` would go to the prev/next 
entry in the jumplist that is from another file than the current one.
```

Original issue reported on code.google.com by `dhahler@gmail.com` on 12 Jun 2015 at 9:57
