Fix the clang-tidy diff SHA for using PR merge (#55318)
Summary:
Since https://github.com/pytorch/pytorch/issues/54967, our clang-tidy CI job has been giving warnings on files that PRs don't touch (see the screenshot below for an example). This PR should fix the issue by comparing against the merge-base of the `merge` commit with `master`, which is just `master` itself.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55318
Test Plan: CI.
Reviewed By: janeyx99
Differential Revision: D27572553
Pulled By: samestep
fbshipit-source-id: 9a833aaeecc2ab22462b3fa99fa3353490c3de85
