D3873: rebase: refactor logic to read rebasestate in a separate function

[Yuya Nishihara]

>   > FWIW, I don't have any better idea to keep old hg not crash with new state
>   >  file, other than:
>   >
>   >   a. use separate file (e.g. "rebasestate2") and leave "rebasestate" in old
>   >      format (or make it an empty file to trigger error.)
>   
>   
>   Yeah, I was preventing this for a long time but feels like I can't anymore.
>   How about having a `.hg/state/` where we will have our new histedit and
>   rebase state files?

A separate directory wouldn't make the situation better. We can't stop writing
a v1 state file anyway since missing v1 state file means "no rebase in progress"
in old hg versions.