Add foldlevel to fugitive_diff_restore

Hi!

I normally have foldlevelstart to 4, and I rarely have stuff folded for long. After Gdiff, which I do extremely often, the level is lowered, but not restored when the diff is finished.

I've just added foldlevel to fugitive_diff_restore (clever idea, BTW), and I've been testing it for a couple of days, opening and closing diff mode in different ways several times.

I'm a bit hesitant to send this pull request, though, since even if the change is trivial, I don't understand how this is something that seems is only been affecting me. It might be that I have such a different setup that doesn't bother others? Is not a huge deal, of course, but it made me do a "zR" after each diff. And I diff all the files one by one after each commit. :-)

Cheers, and thanks a lot for fugitive.