scripts/checkpatch.pl: fix git-show invocation to include diffstat

Without this checkpatch keeps complaining about new/changed files even
when MAINTAINERS has been updated. Normal invocations of checkpatch on
patch files rather than commit IDs are unaffected.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210114165730.31607-13-alex.bennee@linaro.org>