In the common case (sparse files enabled, not reusing old data) we'd
optimise away pulling & writing zero blocks. However in the corner cases
we'd go through the whole processing of pulling the block over the
network which is of course entirely unnecessary.
Now, instead, always take an optimised path for all-zeroes blocks. In
the clean case we do nothing, otherwise we materialise a block of zeroes
and write it directly.
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Signed-off-by: Jakob Borg <jakob@kastelo.net>
f538b47070