Comment 8 for bug 697828

I did some reading into the relevant code in cgit: https://github.com/gitster/git/blob/master/date.c#L604.

If I read the code correctly, the C code interprets "--700" as a double-negative, or equivalent to +0700. I believe that implementation is a mistake, but if copying that mistake into dulwich is the proper fix, I will put together a patch.