scripts/checkpatch: roll diff tweaking into checkpatch itself

Rather than relying on external tweaks lets just do it inside
checkpatch's direct commitish handling which is QEMU specific code
anyway.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20210623102749.25686-4-alex.bennee@linaro.org>