dim: Add --fast option to sparse

Running sparse on each commit in the provided range can be very time
consuming for large series.

With --fast switch we treat the whole range as a single change that gets
checked for any new sparse errors.

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>