[clang-format] Add --staged/--cached option to git-clang-format

When running git-clang-format in a pre-commit hook it's very useful to be able to tell git-clang-format to only look at the --staged/--cached files and not the working directory.

Note this patch is a rebase/fork from {D41147 } which is a fork of {D15465 }

Reviewed By: MyDeveloperDay, HazardyKnusperkeks, lodato

Differential Revision: https://reviews.llvm.org/D90996



Co-authored-by: Mark Lodato <lodato@google.com>