remote-http(s): support SOCKS proxies

With this patch we properly support SOCKS proxies, configured e.g. like
this:

	git config http.proxy socks5://192.168.67.1:32767

Without this patch, Git mistakenly tries to use SOCKS proxies as if they
were HTTP proxies, resulting in a error message like:

	fatal: unable to access 'http://.../': Proxy CONNECT aborted

This patch was required to work behind a faulty AP and scraped from
http://stackoverflow.com/questions/15227130/#15228479 and guarded with
an appropriate cURL version check by Johannes Schindelin.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

@@ -424,6 +424,17 @@ static CURL *get_curl_handle(void)
 
 	if (curl_http_proxy) {
 		curl_easy_setopt(result, CURLOPT_PROXY, curl_http_proxy);
+#if LIBCURL_VERSION_NUM >= 0x071800
+		if (starts_with(curl_http_proxy, "socks5"))
+			curl_easy_setopt(result,
+				CURLOPT_PROXYTYPE, CURLPROXY_SOCKS5);
+		else if (starts_with(curl_http_proxy, "socks4a"))
+			curl_easy_setopt(result,
+				CURLOPT_PROXYTYPE, CURLPROXY_SOCKS4A);
+		else if (starts_with(curl_http_proxy, "socks"))
+			curl_easy_setopt(result,
+				CURLOPT_PROXYTYPE, CURLPROXY_SOCKS4);
+#endif
 	}
 #if LIBCURL_VERSION_NUM >= 0x070a07
 	curl_easy_setopt(result, CURLOPT_PROXYAUTH, CURLAUTH_ANY);