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

[Yuya Nishihara]

>   > I doubt if this would work with --collapse. Can you try writing some tests?
>   
>   What is the expected behaviour in case of --collapse? Should it collapse only those revisions which are rebased, then we may have to ask user to change commit message accordingly.

No idea. Perhaps we can revisit it later by making `hg rebase --stop` just
abort if it was a --collapse session.