code-format: Improve the code-format-helper to be able to run as a git hook (#73957)

As part of #73798 there was some discussion about using the format
helper to run from a git-hook. That was not possible for a number of
reasons, but with these changes it can now be installed as a hook and
then run on the local cache in git instead of a diff between revisions.

This also checks for two environment variables DARKER_FORMAT_PATH and
CLANG_FORMAT_PATH where you can specify the path to the program you want
to use.