Delete bundle file of hg incoming in case of errors, preserve existing files. Let write_bundle always create the bundle file, check if target doesn't exist and delete it in case of errors. This way incoming only has to delete the bundle file if it was meant to be a temporary file.

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