http: support CURLPROXY_HTTPS

HTTP proxy over SSL is supported by curl since 7.52.0.
This is very useful for networks with protocol whitelist.
Signed-off-by: NWei Shuyu <wsy@dogben.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

http.c

@@ -864,6 +864,11 @@ static CURL *get_curl_handle(void)
 		else if (starts_with(curl_http_proxy, "socks"))
 			curl_easy_setopt(result,
 				CURLOPT_PROXYTYPE, CURLPROXY_SOCKS4);
+#endif
+#if LIBCURL_VERSION_NUM >= 0x073400
+		else if (starts_with(curl_http_proxy, "https"))
+			curl_easy_setopt(result,
+				CURLOPT_PROXYTYPE, CURLPROXY_HTTPS);
 #endif
 		if (strstr(curl_http_proxy, "://"))
 			credential_from_url(&proxy_auth, curl_http_proxy);