[clang] [git-clang-format] add fallback style argument option (PR #137609)

[Andrej Pištek via cfe-commits]

apdofficial wrote:

> @llvm/pr-subscribers-clang-format
> 
> Author: Andrej Pištek (apdofficial)
> Changes

Please let me know whether the updated description is sufficient. If noy, I can update it. Thank you.

https://github.com/llvm/llvm-project/pull/137609