topics: fix `hg stack` in case of split This patch fixes the behaviour of hg stack which throws an error in some cases when there are multiple successors. The case of divergence is not handled yet.

tests: add a test showing traceback on `hg stack` in case of split The traceback will be fixed in the next patch.

branching: merge stable into default

test-compat: merge mercurial-4.2 into mercurial-4.1

test-compat: merge mercurial-4.3 into mercurial-4.2

test-compat: merge stable into mercurial-4.3

changelog: add an entry for the split+branch fix The world needs to know.

branching: merge with stable

split: force the branch to fix the split bug Also restore it after the split

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.

stablesort: simplify processing of non-merge changesets As long as a changeset as only one parent, the next element in the sort (starting from the head) has to be that parent. We detect and take advantage of that shortcut in the non-merge case.

stablesort: extract a '_revsfrom' method Walking from the revision is usually simple unless the revision is a merge. Having this walking logic isolated will help us refine it.

stablerange: use the new cache object in the 'mergepoint' version Every improvement to the cache should reflect on the stable range from there. Since we no longer use the basic sort function, we can simplify the inheritance by dropping one layer.