http: support CURLOPT_PROTOCOLS_STR

http: support CURLOPT_PROTOCOLS_STR

The CURLOPT_PROTOCOLS (and matching CURLOPT_REDIR_PROTOCOLS) flag was
deprecated in curl 7.85.0, and using it generate compiler warnings as of
curl 7.87.0. The path forward is to use CURLOPT_PROTOCOLS_STR, but we
can't just do so unilaterally, as it was only introduced less than a
year ago in 7.85.0.

Until that version becomes ubiquitous, we have to either disable the
deprecation warning or conditionally use the "STR" variant on newer
versions of libcurl. This patch switches to the new variant, which is
nice for two reasons:

  - we don't have to worry that silencing curl's deprecation warnings
    might cause us to miss other more useful ones

  - we'd eventually want to move to the new variant anyway, so this gets
    us set up (albeit with some extra ugly boilerplate for the
    conditional)

There are a lot of ways to split up the two cases. One way would be to
abstract the storage type (strbuf versus a long), how to append
(strbuf_addstr vs bitwise OR), how to initialize, which CURLOPT to use,
and so on. But the resulting code looks pretty magical:

  GIT_CURL_PROTOCOL_TYPE allowed = GIT_CURL_PROTOCOL_TYPE_INIT;
  if (...http is allowed...)
	GIT_CURL_PROTOCOL_APPEND(&allowed, "http", CURLOPT_HTTP);

and you end up with more "#define GIT_CURL_PROTOCOL_TYPE" macros than
actual code.

On the other end of the spectrum, we could just implement two separate
functions, one that handles a string list and one that handles bits. But
then we end up repeating our list of protocols (http, https, ftp, ftp).

This patch takes the middle ground. The run-time code is always there to
handle both types, and we just choose which one to feed to curl.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>