ghstack command won't work if some PR has been closed

[zasdfgbnm]

I am trying to modify pytorch/pytorch#18650 (comment) by checking out the corresponding commits in master branch, modify the file, and git commit --amend and then run ghstack, but I get an error:

# Updating https://github.com/pytorch/pytorch/pull/18648
ERROR: Fatal exception
Traceback (most recent call last):
  File "/home/gaoxiang/.virtualenvs/pt/lib/python3.7/site-packages/ghstack/__main__.py", line 153, in main
    update_fields=args.update_fields
  File "/home/gaoxiang/.virtualenvs/pt/lib/python3.7/site-packages/ghstack/submit.py", line 153, in main
    submitter.post_process()
  File "/home/gaoxiang/.virtualenvs/pt/lib/python3.7/site-packages/ghstack/submit.py", line 594, in post_process
    base=s.base)
  File "/home/gaoxiang/.virtualenvs/pt/lib/python3.7/site-packages/ghstack/github.py", line 50, in patch
    return self.rest('patch', path, **kwargs)
  File "/home/gaoxiang/.virtualenvs/pt/lib/python3.7/site-packages/ghstack/github_real.py", line 98, in rest
    r.raise_for_status()
  File "/home/gaoxiang/.virtualenvs/pt/lib/python3.7/site-packages/requests/models.py", line 939, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 422 Client Error: Unprocessable Entity for url: https://api.github.com/repos/pytorch/pytorch/pulls/18648

I think this is because the corresponding PR (pytorch/pytorch#18648) has been landed and closed.

In such case, instead of failing with error, should we just skip modifying that PR and continue?

[ezyang]

Fixed on head, please give it a try!

[zasdfgbnm]

Tested to be working. Thanks!