http: Update httpclient options when reusing an existing connection.

Httpclient internally stores a copy of the certificate_check callback and
payload. When connecting via HTTPS, and if the server sends back
"Connection: close" after the first request, the following request would
attempt to re-use the httpclient and call the (now outdated) callback. In
particular for pygit2 this is a problem, since callbacks / payloads are only
valid for the duration of a libgit2 call, leading to a ffi.from_handle()
error and crashing the Python interpreter.

src/libgit2/transports/http.c

@@ -655,6 +655,7 @@ static int http_action(
 {
 	http_subtransport *transport = GIT_CONTAINER_OF(t, http_subtransport, parent);
 	git_remote_connect_options *connect_opts = &transport->owner->connect_opts;
+	git_http_client_options opts = {0};
 	http_stream *stream;
 	const http_service *service;
 	int error;
@@ -683,14 +684,14 @@ static int http_action(
 	stream = git__calloc(sizeof(http_stream), 1);
 	GIT_ERROR_CHECK_ALLOC(stream);
 
-	if (!transport->http_client) {
-		git_http_client_options opts = {0};
-
-		opts.server_certificate_check_cb = connect_opts->callbacks.certificate_check;
-		opts.server_certificate_check_payload = connect_opts->callbacks.payload;
-		opts.proxy_certificate_check_cb = connect_opts->proxy_opts.certificate_check;
-		opts.proxy_certificate_check_payload = connect_opts->proxy_opts.payload;
+	opts.server_certificate_check_cb = connect_opts->callbacks.certificate_check;
+	opts.server_certificate_check_payload = connect_opts->callbacks.payload;
+	opts.proxy_certificate_check_cb = connect_opts->proxy_opts.certificate_check;
+	opts.proxy_certificate_check_payload = connect_opts->proxy_opts.payload;
 
+	if (transport->http_client) {
+		git_http_client_set_options(transport->http_client, &opts);
+	} else {
 		if (git_http_client_new(&transport->http_client, &opts) < 0)
 			return -1;
 	}

src/libgit2/transports/httpclient.c

@@ -1541,6 +1541,15 @@ int git_http_client_new(
 	return 0;
 }
 
+/* Update the options of an existing httpclient instance. */
+void git_http_client_set_options(
+	git_http_client *client,
+	git_http_client_options *opts)
+{
+	if (opts)
+		memcpy(&client->opts, opts, sizeof(git_http_client_options));
+}
+
 GIT_INLINE(void) http_server_close(git_http_server *server)
 {
 	if (server->stream) {

src/libgit2/transports/httpclient.h

@@ -88,6 +88,16 @@ extern int git_http_client_new(
 	git_http_client **out,
 	git_http_client_options *opts);
 
+/**
+ * Update the options of an existing httpclient instance.
+ *
+ * @param client the httpclient instance to modify
+ * @param opts new options or NULL to keep existing options
+ */
+extern void git_http_client_set_options(
+	git_http_client *client,
+	git_http_client_options *opts);
+
 /*
  * Sends a request to the host specified by the request URL.  If the
  * method is POST, either the content_length or the chunked flag must