Fixing numstat parsing for cases where files have spaces in their filepath

I've also fixed the fixture in "test/fixtures/log" because the added/deleted stats are separated by spaces there, although git outputs the numstat information separated by tabs, as you can see here:

https://github.com/git/git/blob/b91766295f2b873bbd4ef79c06de05aff27f8e12/diff.c#L1524