pytorch

9f890a92 - make sure clang-tidy is diffing against the right thing (#28788)

make sure clang-tidy is diffing against the right thing (#28788) Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/28788 Okay, my last fix was wrong because it turns out that the base SHA is computed at PR time using the actual repo's view of the base ref, not the user's. So if the user doesn't rebase on top of the latest master before putting up the PR, the diff thing is wrong anyway. This PR fixes the issue by not relying on any of these API details and just getting the merge-base of the base and head refs, which should guarantee we are diffing against the right thing. This solution taken from https://github.com/github/VisualStudio/pull/1008 Test Plan: Imported from OSS Differential Revision: D18172391 Pulled By: suo fbshipit-source-id: 491a50119194508b2eefa5bd39fe813ca85f27b1