Update `syncbranches` workflow (#71420)
Summary:
Use `pytorchmergebot` credentials to do the merge
Infer sync branch name from the workflow rather than hardcode it
Move common functions from `syncbranches.py` to `gitutils.py`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71420
Reviewed By: bigfootjon
Differential Revision: D33638846
Pulled By: malfet
fbshipit-source-id: a568fd9ca04f4f142a7f5f64363e9516f5f4ef1c
