http: factor out http error code handling

http: factor out http error code handling

Most of our http requests go through the http_request()
interface, which does some nice post-processing on the
results. In particular, it handles prompting for missing
credentials as well as approving and rejecting valid or
invalid credentials. Unfortunately, it only handles GET
requests. Making it handle POSTs would be quite complex, so
let's pull result handling code into its own function so
that it can be reused from the POST code paths.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>