dim: use git show --no-use-mailmap for checkpatch

As we checkpatch the applied commits rather than the patches, avoid
mailmap conversions, if any. Otherwise we might get complaints about
author Signed-off-by missing.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>