chunkiter: handle large reads more efficiently - for large reads, don't attempt to read more than necessary - if we've gathered the exact number of bytes needed, avoid a string copy

#!/bin/sh

mkdir t
cd t
hg init
echo a > a
hg add a
hg commit -m test -d "1000000 0"
rm .hg/requires
hg tip
echo indoor-pool > .hg/requires
hg tip

true