Use _concat for repartition

[fjetter]

Closes #625

I don't really like _concat. Starting with the name of it and it's relatively magical rv. It does fix the problem.

FWIW I encountered this in #294 during division calculation since the optimization is now different and includes a postcompute, i.e. a repartition

[fjetter]

I suspect that we'd have to use this in other places as well if this is what we want do to

[phofl]

See my comment in the issue, this solves this particular case, but having integers in partitions is a general issue that is bad

[fjetter]

Closing in favor of dask/dask#10739