[git-clang-format] Add --diffstat parameter

[git-clang-format][PR46815] Add diffstat functionality

Adding a --diffstat parameter to git-clang-format that essentially uses git diff --stat, i.e. lists the files needing
formatting. This is useful for CI integration or manual usage where one wants to list the files not properly formatted.

I use it for the Suricata project's github action (CI) integration that verifies proper formatting of a pull request
according to project guidelines where it's very helpful to say which files are not properly formatted. I find the list
of files much more useful than e.g. showing the diff in this case using git-clang-format --diff.

An alternative would be to take an additional parameter to diff, e.g. git-clang-format --diff --stat

The goal is not to provide the whole git diff --stat=... parameter functionality, just plain git diff --stat.

Reviewed By: MyDeveloperDay, JakeMerdichAMD

Differential Revision: https://reviews.llvm.org/D84375