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

[yuja (Yuya Nishihara)]

yuja added a comment.


  >   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
  
  The code looks good to me. We'll need to update `cmdutil.unfinishedstates`
  as well.

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D3668

To: pulkit, #hg-reviewers
Cc: yuja, mercurial-devel