scripts/ci/check_compliance: add GitDiffCheck compliance check

This commit adds a new `GitDiffCheck` compliance check that checks the
newly added commits with `git diff --check` and reports them back if an
error is found.

This check is needed for some files (e.g. Kconfig) as they are not checked
by `Kconfig` and `KconfigBasic` checks on every commit in a pull request.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>