D3753: graft: store the node from where we started the operation in graftstate

[Yuya Nishihara]

> --- a/mercurial/commands.py
> +++ b/mercurial/commands.py
> @@ -2233,6 +2233,8 @@
>              raise error.Abort(_('no revisions specified'))
>          cmdutil.checkunfinished(repo)
>          cmdutil.bailifchanged(repo)
> +        # the node from where we started the graft operation
> +        statedata['startnode'] = repo['.'].node()

Nit: Isn't it always the parent of the `newnodes[0]`?

I don't have strong opinion whether we should store the startnode, but if
we store it at the start of the graft, I think we won't need to care for
`statedata.get('newnodes') is None` case, in the first patch. I mean we can
always record newnodes no matter if it may be incomplete or not since the
existence of the startnode gives us a trust.