Receive: Use explicit discards in sparse mode

[tasket]

Sparse writes to disk are currently only partially implemented, as writing sparsely over an existing file/volume presents a special problem in Python. Specifically, de-allocating old data that is being replaced with a zeroed-out chunk (e.g. where seeking ahead isn't an option) isn't supported and the current implementation simply writes zeroes there.

Using fcntl lib (or possibly ctypes) may be necessary to remove this limitation.

[tasket]

See commit f6644b9