D3959: rebase: add --stop option to stop rebase at any point (issue5206)

[Yuya Nishihara]

> +    def _stoprebase(self):
> +        """stop the interrupted rebase"""
> +        self.restorestatus()
> +        if not self.stateobj.exists():
> +            raise error.Abort(_("no interrupted rebase found"))
> +        allowunstable = obsolete.isenabled(self.repo, obsolete.allowunstableopt)
> +        if not (self.keepf or allowunstable):
> +            raise error.Abort(_("can't remove original changesets with"
> +                  " unrebased descendants"),
> +                hint=_('either enable evolve extension to allow unstable '
> +                       'revisions or use --keep to keep original changesets'))
> +
> +        # update to the last rebased node if any
> +        ctx = self.repo[None]
> +        pars = [p.node() for p in ctx.parents()]
> +        p1 = pars[0]
> +        hg.updaterepo(self.repo, p1, overwrite=True)

Perhaps this differs from what a successful rebase would do. Unlike graft,
`hg rebase` goes back to the original working directory.

> +        # either mark obsolete or keep rebased revisions
> +        repo, ui, opts = self.repo, self.ui, self.opts
> +        fm = ui.formatter('rebase', opts)
> +        fm.startitem()
> +        clearrebased(ui, repo, self.destmap, self.state, self.skipped,
> +                     collapsedas=None, keepf=self.keepf, fm=fm)
> +        clearstatus(self.repo)
> +        fm.end()
> +        return 0

I doubt if this would work with --collapse. Can you try writing some tests?

I'm not pretty sure, but it might be possible to utilize `_finishrebase()`
to implement --stop, roughly by:

1. restore status
2. drop revisions that aren't rebased yet
3. finish it

> +    elif stop:
> +        #todo: raise error for conflicting options

Yes!