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

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 <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Willian Rampazzo <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20210114165730[email protected]>