add support for streaming clone. existing clone code uses pull to get changes from remote repo. is very slow, uses lots of memory and cpu. new clone code has server write file data straight to client, client writes file data straight to disk. memory and cpu used are very low, clone is much faster over lan. new client can still clone with pull, can still clone from older servers. new server can still serve older clients.

#!/bin/sh

hg init
touch unknown

touch a
hg add a
hg ci -m "1" -d "1000000 0"

touch b
hg add b
hg ci -m "2" -d "1000000 0"

echo %% Should show unknown
hg status
hg revert -r 0
echo %% Should show unknown and b removed
hg status
echo %% Should show a and unknown
ls