use base sha in clang-tidy instead of base ref (#28388)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28388
The clang-tidy script diffs the PR head ref against the base ref so that
it works only on changed lines. If the base ref is a stale `master`,
then the script will fetch upstream `master` and potentially report
unrelated changes in the diff
Use the base sha instead of ref so that the revision that the script
diffs against is stable.
Test Plan: Imported from OSS
Differential Revision: D18051363
Pulled By: suo
fbshipit-source-id: 80ead2f837e2d6244245ed7b576e84a99f0ea035
