remote-curl: Fix warning after HTTP failure

remote-curl: Fix warning after HTTP failure

If the HTTP connection is broken in the middle of a fetch or clone
body, the client presented a useless error message due to part of
the upload-pack->remote-curl pkt-line protocol leaking out of the
helper as the helper's "fetch result":

  error: RPC failed; result=18, HTTP code = 200
  fatal: The remote end hung up unexpectedly
  fatal: early EOF
  fatal: unpack-objects failed
  warning: https unexpectedly said: '0000'

Instead when the HTTP RPC fails discard all remaining data from
upload-pack and report nothing to the transport helper. Errors
were already sent to stderr.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>