[GHF] Update commit body generation for GHF PRs (#80105)
Use the same logic regular PR (i.e. extract commit title and body from
PR description rather than from original commit)
According to @ezyang this is how it's supposed to be
Tested in https://github.com/malfet/deleteme/pull/27
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80105
Approved by: https://github.com/janeyx99
