script: Re-enable clang-format preupload check with our own script

git-clang-format with --diff in Debian Testing always return 1, while
the useful diff printout is in stdout. Fork out AOSP's clang-format.py
so we can run git-clang-format in repo pre-upload.

Change-Id: Ief24406d4f60ae46532572729c3c9f6d1c5fff67