Fix xxd output to be same on mac and linux.
Fix xxd output to be same on mac and linux.

Summary:
Fix xxd output to be same on mac and linux, allows tests
to pass on both mac and linux with the same data.

Reviewed By: dariorussi

Differential Revision: D5042027

fbshipit-source-id: 628747d8f328655be4822306b519a19161826302

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