[mergebot] rebase + merge (#85028)
adds flag to rebase and merge with one command by just running the tryrebase.py script before running the trymerge.py script
testing on https://github.com/clee2000/random-testing/pull/19
corresponding test-infra change: https://github.com/pytorch/test-infra/pull/712
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85028
Approved by: https://github.com/janeyx99, https://github.com/huydhn
