Return proper HTTP response when accessing info/

Currently, when a user directly accesses the info command of a
repository, we exit cgit without printing anything to stdout, bringing
up error messages like "502 Bad Gateway" or "An error occurred while
reading CGI reply (no response received)". Instead of bailing out, at
least print the HTTP headers, including a reasonable error message.

Reported-by: Janus Troelsen
Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>

ui-clone.c

@@ -71,8 +71,10 @@ static void send_file(char *path)
 
 void cgit_clone_info(void)
 {
-	if (!ctx.qry.path || strcmp(ctx.qry.path, "refs"))
+	if (!ctx.qry.path || strcmp(ctx.qry.path, "refs")) {
+		html_status(400, "Bad request", 0);
 		return;
+	}
 
 	ctx.page.mimetype = "text/plain";
 	ctx.page.filename = "info/refs";