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.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_*() */