`ghstack submit --update-fields` created a new pull request if the ghstack info is damaged or missing

[rec]

This might not be expected behavior. Two other possibilities are that ghstack submit --update-fields fails if the ghstack info in the commit message is damaged or missing, or to create another flag --update-only/-U, with this behavior.

I'd be willing to submit a pull request for either one of these behaviors if there were interest.

This happened when I restored my ghstack info from a branch as:

    ghstack-source-id: de78c83d0763391c27c41501f614fafd49c9395c
    Pull Request resolved: #128416

instead of

    ghstack-source-id: de78c83d0763391c27c41501f614fafd49c9395c
    Pull Request resolved: https://github.com/pytorch/pytorch/pull/128416

[ezyang]

It seems fine to have update fields chicken out if it thinks it needs to make a new PR. Send a patch?