[BE][GHF] Add `retries_decorator` (#101227)
I've noticed that 3-4 functions in trymerge are trying to implement similar tail recursion for flaky network retries.
Unify them using single wrapper in `gitutils.py`
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### <samp>🤖 Generated by Copilot at 8d40631</samp>
> _`retries_decorator`_
> _adds resilience to GitHub scripts_
> _autumn of errors_
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101227
Approved by: https://github.com/kit1980
