Merge branch 'dl/remote-curl-deadlock-fix'

On-the-wire protocol v2 easily falls into a deadlock between the
remote-curl helper and the fetch-pack process when the server side
prematurely throws an error and disconnects.  The communication has
been updated to make it more robust.

* dl/remote-curl-deadlock-fix:
  stateless-connect: send response end packet
  pkt-line: define PACKET_READ_RESPONSE_END
  remote-curl: error on incomplete packet
  pkt-line: extern packet_length()
  transport: extract common fetch_pack() call
  remote-curl: remove label indentation
  remote-curl: fix typo