jumplist: go to prev/next entry from another file

[GoogleCodeExporter]

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