Fix bugs in new diff patch code

Fix bugs in new diff patch code

This fixes all the bugs in the new diff patch code.  The only
really interesting one is that when we merge two diffs, we now
have to actually exclude diff delta records that are not supposed
to be tracked, as opposed to before where they could be included
because they would be skipped silently by `git_diff_foreach()`.
Other than that, there are just minor errors.