split: add a test demonstrating that split doesn't take the right branch Also add a test to check that the dirstate is in the right state in case the user abort the split.

stablerange: compute the subrange closure on an unfiltered repository This avoid various overhead from filtering.

stablesort: write a flat version of the algorithm This new version never needs to iterate over the full repository even when merge are encountered. Having the algorithm flat also prepare caching works since the iteration sequence can we expressed with a couple of flat loop.