rebase via comment - ghstack commit author fix (#80747)
fixes #80635
tested on pytorch-canary: https://github.com/pytorch/pytorch-canary/pull/115
steal the identity of the committer of the commit on the orig branch instead of overwriting it with mergebot's identity
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80747
Approved by: https://github.com/malfet, https://github.com/seemethere
