Fix reduce for predecessor.

The assumption was that the predecessor, which is a distributed vertex descriptor, should be
the same as the key. The key is actually a local vertex descriptor, and these two types may
not match.

Signed-off-by: Marcin Zalewski <marcin.zalewski@gmail.com>
Merge pull request #4 from marcinz/develop

Fix reduce for predecessor.

Right, I see that now, thanks for the patch.