fix clang-tidy failing all the time on random lines (#25078)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25078
Our script is set up to only run on lines generated by diffing your branch against the base branch.
But we were using `$TRAVIS_BRANCH` to refer to the target branch, which was causing the script to diff against master, generating many spurious lines of diff output to be clang-tidy'd
Test Plan: Imported from OSS
Differential Revision: D16993054
Pulled By: suo
fbshipit-source-id: 7bffa890f6a1a2d5566ef01b9798c4eb86d8169f
