When the http server gives an incomplete response to a smart-http
rpc call, it could lead to client waiting for a full response that
will never come.  Teach the client side to notice this condition
and abort the transfer.

An improvement counterproposal has failed.
cf. <20161114194049.mktpsvgdhex2f4zv@sigill.intra.peff.net>

* dt/smart-http-detect-server-going-away:
  upload-pack: optionally allow fetching any sha1
  remote-curl: don't hang when a server dies before any output