D3668: graft: add a new `--stop` flag to stop interrupted graft

[Yuya Nishihara]

>   In https://phab.mercurial-scm.org/D3668#58070, @yuja wrote:
>   
>   > >   The `--stop` flag takes back you to the last successful step i.e. it will keep
>   > >   your grafted commits, it will just clear the mergestate and interrupted graft
>   > >   state.
>   >
>   > No idea if git is right regarding UI, but they appear to have `--quit` for
>   >  this.
>   >
>   > https://git-scm.com/docs/git-cherry-pick
>   
>   I asked people on IRC, nbjoerg and one more said '--stop' is better. However I am fine with changing that to `--quit`

Good. Since I don't have any preference either, let's just stick to --stop.

Replaced some --abort in error message and queued, thanks.