speed up rebuild in postgres

[morus]

The change simply exempts all records that will not change in the update.

Since reorder_with_parent_id is called a lot for rebuild! this leads to a huge performance boost for our data.
(many small trees, we have some 10k records and 4k top level elements)

Great tests by the way. My first solution got the check wrong with respect to null values (!= instead of is distinct from)...

I ran the tests for postgres only and one rails version, but the nature of the change should allow that.

[mceachen]

Thanks!

[glytch]

Thank you! I sat down to fix this very problem today and now I can spend the day doing something else :)