Bugfix: Correctly detect test changes in PRs (#101304)
Fixes a bug where the logic for deciding what tests have been edited by a PR would include all files that had been edited since the merge base, including files that were in main!
Now it will only consider the files that are part of the PR itself
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101304
Approved by: https://github.com/seemethere, https://github.com/malfet
