hooks: add preupdate and update hooks.
preupdate run before working dir updated. can prevent update.
update run after working dir updated.
port:4:exportport:4:vaportightport:4:import/exportFIXME: history is wrong hereport:1:1:-:eggs:importport:1:2:+:eggs:vaportightport:1:3:+:eggs:import/exportport:0:2:+:spam:exportport:0:1:+:spam:importport:4:import/export