Extend pre-commit hook to include clang-format

[cmannett85]

Currently we're relying on the IDEs to do it all for us and then have it checked upstream. But some IDEs are a bit flaky (I'm looking at you VS), so it'd be good to do this from the pre-commit.

Rules:

• Only do it for the files that have changed
• Fail the check, do not try to format the files
• Be aware that clang-format may not be on the path