Work around curl-gnutls not liking to be reinitialized

Work around curl-gnutls not liking to be reinitialized

curl versions 7.16.3 to 7.18.0 included had a regression in which https
requests following curl_global_cleanup/init sequence would fail with ASN1
parser errors with curl-gnutls. Such sequences happen in some cases such
as git fetch.

We work around this by removing the http_init and http_cleanup calls from
get_refs_via_curl, replacing them with a transport->data initialization
with the http_walker (which does http_init).

While the http_walker is not currently used in get_refs_via_curl, http
and walker code refactor will make it use it.

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>