Merge branch 'jk/receive-pack-unpack-error-to-pusher' into maint

"git receive-pack" (the counterpart to "git push") did not give
progress output while processing objects it received to the puser
when run over the smart-http protocol.

* jk/receive-pack-unpack-error-to-pusher:
  receive-pack: drop "n/a" on unpacker errors
  receive-pack: send pack-processing stderr over sideband
  receive-pack: redirect unpack-objects stdout to /dev/null