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.

port:4:export
port:4:vaportight
port:4:import/export
FIXME: history is wrong here
port:1:1:-:eggs:import
port:1:2:+:eggs:vaportight
port:1:3:+:eggs:import/export
port:0:2:+:spam:export
port:0:1:+:spam:import
port:4:import/export