Add -Z option to zero free space included in the image due to -F.
Previously, this "internal free space" was whatever garbage happened to be on the disk we were imaging. By zeroing, we eliminate any leakage of information from the source disk and also allow the image to compress just a tad (1-4%) better. Why isn't this the default? Eh...no good reason, other than that this whole squish-out-small-free-ranges-to-allow-longer-writes optimization should be done by the client and should not be encoded in the image.
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