Add workflow to sync `fbsync`->`master` (#71013)
Summary:
Main logic of the workflow is implemented in `syncbranches.py` script,
which computes patch-id's of divergent history (as determined by `git
merge-base`) and treats all patches present in sync branch with
non-matching patch-ids as ones missing in target branch
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71013
Reviewed By: bigfootjon
Differential Revision: D33480885
Pulled By: malfet
fbshipit-source-id: bd72c061720d0cba49c6754ec4e94437d8a5c262
