rebase: filter out descendants of divergence-causing commits earlier `hg rebase` treats obsolete commits differently depending what has happened to the commit: 1) Obsolete commit without non-obsolete successors: Skipped, and a note is printed ("it has no successor"). 2) Obsolete commit with a successor in the destination (ancestor of it): Skipped, and a note is printed ("already in destination"). 3) Obsolete commit with a successor in the rebase set: The commit and its descendants are skipped, and a note is printed ("not rebasing <commit> and its descendants as this would cause divergence"), unless `allowdivergence` config set. 4) Obsolete commit with a successor elsewhere: Error ("this rebase will cause divergences"), unless `allowdivergence` config set. Before this patch, we did all those checks up front, except for (3), which was checked later. The later check consisted of two parts: 1) filtering out of descendants, and 2) conditionally printing message if the `allowdivergence` config was not set. This patch makes it so we do the filtering early. A consequence of filtering out divergence-causing commits earlier is that we rebase commits in slightly different order, which has some impact on tests. Differential Revision: https://phab.mercurial-scm.org/D10249