ci: Add note about file_diff_from_base for GHA (#60110)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60110
file_diff_from_base is currently bugged for ghstack PRs since it fails
to find a merge base
Signed-off-by: Eli Uriegas <eliuriegas@fb.com>
Test Plan: Imported from OSS
Reviewed By: driazati
Differential Revision: D29168767
Pulled By: seemethere
fbshipit-source-id: 580a909aa392541769cbbfdc6acce1e6c5d1c341
