patch, 6 partsSideband support in push (send-pack/receive-pack)

This series adds the same sort of sideband channel we use in fetch (fetch-pack/upload-pack) to pass progress messages back to the client from the server side.

  1)  Allow run_command stdout_to_stderr when stderr is a pipe
  2)  Automatically close stderr pipes created by run_command
  3)  Refactor packet_write to prepare to integrate with sideband
  4)  Refactor send-pack/receive-pack capability handshake for extension
  5)  Add side-band-64k extension to send-pack/receive-pack for hook output
  6)  Redirect receive-pack hook output into sideband channel

I didn't really see any comments from the list on this topic the last time I posted it for comment, so here is a rebased version on current master.