http: allow providing extra headers for http requests

Add a way for callers to request that extra headers be included when
making http requests.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

@@ -1723,6 +1723,14 @@ static int http_request(const char *url,
 
 	headers = curl_slist_append(headers, buf.buf);
 
+	/* Add additional headers here */
+	if (options && options->extra_headers) {
+		const struct string_list_item *item;
+		for_each_string_list_item(item, options->extra_headers) {
+			headers = curl_slist_append(headers, item->string);
+		}
+	}
+
 	curl_easy_setopt(slot->curl, CURLOPT_URL, url);
 	curl_easy_setopt(slot->curl, CURLOPT_HTTPHEADER, headers);
 	curl_easy_setopt(slot->curl, CURLOPT_ENCODING, "gzip");

http.h

@@ -172,6 +172,13 @@ struct http_get_options {
 	 * for details.
 	 */
 	struct strbuf *base_url;
+
+	/*
+	 * If not NULL, contains additional HTTP headers to be sent with the
+	 * request. The strings in the list must not be freed until after the
+	 * request has completed.
+	 */
+	struct string_list *extra_headers;
 };
 
 /* Return values for http_get_*() */