Do not unnecessarily calculate crc for composite parts.
Do not unnecessarily calculate crc for composite parts.

dtjackson/gsutil,ttiurani/gsutil,BrandonY/gsutil,chriskuehl/gsutil-debian,GoogleCloudPlatform/gsutil,mattdr/gsutil,fishjord/gsutil,GoogleCloudPlatform/gsutil,dimfeld/gsutil,craigcitro/gsutil