Merge branch 'sp/maint-push-sideband' into maint-1.6.6

Merge branch 'sp/maint-push-sideband' into maint-1.6.6

* sp/maint-push-sideband:
  receive-pack: Send internal errors over side-band #2
  t5401: Use a bare repository for the remote peer
  receive-pack: Send hook output over side band #2
  receive-pack: Wrap status reports inside side-band-64k
  receive-pack: Refactor how capabilities are shown to the client
  send-pack: demultiplex a sideband stream with status data
  run-command: support custom fd-set in async
  run-command: Allow stderr to be a caller supplied pipe