Make sure the marks are not written out when the transport helper
did not finish happily, to avoid leaving a marks file that is out of
sync with the reality.
* fc/transport-helper-sync-error-fix:
t5801 (remote-helpers): cleanup environment sets
transport-helper: fix sync issue on crashes
transport-helper: trivial cleanup
transport-helper: propagate recvline() error pushing
remote-helpers: make recvline return an error
transport-helper: remove barely used xchgline()