Add last missing script for format check in github (#23)

Summary:
Adding `scripts/git-clang-format`, which is used for format checks in github. The scripts are forked from the OSS CI scripts from Velox.

Pull Request resolved: #23

Reviewed By: zzhao0

Differential Revision: D54615300

Pulled By: pedroerp

fbshipit-source-id: 38161fb3aad5dc8c553a04398a334fa8fc6ff8db