Merge branch 'fc/transport-helper-sync-error-fix'

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()