split: new extension to split changesets This diff introduces an experimental split extension to split changesets. The implementation is largely inspired by Laurent Charignon's implementation for mutable-history (changeset 9603aa1ecdfd54b0d86e262318a72e0a2ffeb6cc [1]) This version contains various improvements: - Rebase by default. This is more friendly for new users. Split won't lead to merge conflicts so a rebase won't give the user more trouble. This has been on by default at Facebook for months now and seems to be a good UX improvement. The rebase skips obsoleted or orphaned changesets, which can avoid issues like allowdivergence, merge conflicts, etc. This is more flexible because the user can decide what to do next (see the last test case in test-split.t) - Remove "Done split? [y/n]" prompt. That could be detected by checking `repo.status()` instead. - Works with obsstore disabled. Without obsstore, split uses strip to clean up old nodes, and it can even handle split a non-head changeset with "allowunstable" disabled, since it runs a rebase to solve the "unstable" issue in a same transaction. - More friendly editor text. Put what has been already split into the editor text so users won't lost track about where they are. [1]: https://bitbucket.org/marmoute/mutable-history/commits/9603aa1ecdfd54b Differential Revision: https://phab.mercurial-scm.org/D1082