First block of file is sparse

[fadden]

Looking at Total Replay II v1.0-alpha.4.hdv:

W First block of sapling file is sparse: X/AMER.CHALL.1
W First block of tree file is sparse: X/A2.SD1
W First block of tree file is sparse: X/A2.SD11
W First block of tree file is sparse: X/A2.SD13
W First block of tree file is sparse: X/A2.SD14B
W First block of tree file is sparse: X/A2.SD2
W First block of tree file is sparse: X/A2.SD3
W First block of tree file is sparse: X/A2.SD4
W First block of tree file is sparse: X/A2.SD5
W First block of tree file is sparse: X/A2.SD6
W First block of tree file is sparse: X/A2.SD7A
W First block of tree file is sparse: X/A2.SD7B
W First block of tree file is sparse: X/A2.SDS1

See mach-kernel/cadius#42 for a proposed fix (and fadden/ciderpress#49 for some examples of how this can be a problem).