Fix :!! message window command line string.

The ! should be expanded to the previous command string.

common/content/io.js

@@ -374,6 +374,7 @@ function IO() //{{{
 
             let output = io.system(arg);
 
+            commandline.command = "!" + arg;
             commandline.echo(template.commandOutput(<span highlight="CmdOutput">{output}</span>));
 
             autocommands.trigger("ShellCmdPost", {});