Merge branch 'jx/t5411-flake-fix'

The exchange between receive-pack and proc-receive hook did not
carefully check for errors.

* jx/t5411-flake-fix:
  receive-pack: use default version 0 for proc-receive
  receive-pack: gently write messages to proc-receive
  t5411: new helper filter_out_user_friendly_and_stable_output