merge back with 3.3 compat branch

tests: adapt the test for Mercurial 3.3 Some fix in the obsolete parent directory warning where also necessary.

merge with stable Test are updated to run with current mercurial stable (3.4.1)

next: add a --evolve option When on a topological head, this option will trigger the evolution of a unstable changeset that will result in a children of the current working copy parent. This should ease stacked changesets workflow by allowing to stick to prev and next to move through a stack of diff, evolving part of it on demand when needed. In case of ambiguity, the command will ask the user to choose. We need a better definition of "the stack of changesets I'm working on" to be able to seamlessly handling branching.

next: print some message when on a head with some aspiring children Being on a head does not necessary means that this is the end of the road. It is likely that some changesets could be evolve as children. We detect this case and provide a hint.

next: reword error output We move to a more compact and direct phrasing. I'm not fully happy about it yet, but this is a step forward. The listing of candidate should maybe be moved behind a --verbose flag (and such verbose flag be suggested in the error). G: changed tests/test-prev-next.t

next: refactor the command code We make the conditional flatter and the return more straight forward. This will make addition of more complex cases more straightforward in future changesets.

next: add extensive testing to the 'next' command We are going to introduce more complexity to it. We add the full extend of the case we want tested to see how they evolve later.

readme: add an entry about the obsolete wdir parent hint

evolve: add a hint when the parent working copy become obsolete This should help people find the right commands to recover from this situation.

evolve: better error message when command is Ambiguous Having more data help the user.

evolve: mechanism to load some commands selectively This patch introduces a new config option: experimental.evolutioncommands to load evolve's command selectively. It is part of a sequence of patches to make evolve's command respect the value of experimental.evolution. Once these commands are ready and actually respect the flag, they are safe to use and can be enabled with the mechanism developed in this patch.