When determining diffs between two iterators we may need to
recurse into an unmatched directory for the "new" iterator when
it is either a prefix to the current item of the "old" iterator
or when untracked/ignored changes are requested by the user and
the directory is untracked/ignored.

When advancing into the directory and no files are found, we will
get back `GIT_ENOTFOUND`. If so, we simply skip the directory,
handling resulting unmatched old items in the next iteration. The
other case of `iterator_advance_into` returning either
`GIT_NOERROR` or any other error but `GIT_ENOTFOUND` will be
handled by the caller, which will now either compare the first
directory entry of the "new" iterator in case of `GIT_ENOERROR`
or abort on other cases.

Improve readability of the code to make the above logic more
clear.