Fix wrong committer when rebase and merge (#91330)
When using in the context of the merge workflow, the committer's name and email have already been set as part of the workflow, i.e. https://github.com/pytorch/pytorch/actions/runs/3754075933/jobs/6377965897:
```
git config --global user.email "pytorchmergebot@users.noreply.github.com"
git config --global user.name "PyTorch MergeBot"
```
Trying to overwrite this in tryrebase's ghstack logic would lead to the wrong committer showing up. The fix check if the email and name have already been set so that the code doesn't overwrite them.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91330
Approved by: https://github.com/kit1980, https://github.com/clee2000, https://github.com/malfet
