debugger: repeat command functionality

lib/_debugger.js

@@ -351,10 +351,15 @@ function Interface() {
   });
 
   term.on('line', function(cmd) {
-
     // trim whitespace
     cmd = cmd.replace(/^\s*/, '').replace(/\s*$/, '');
-    self.handleCommand(cmd);
+
+    if (cmd.length) {
+      self._lastCommand = cmd;
+      self.handleCommand(cmd);
+    } else {
+      self.handleCommand(self._lastCommand);
+    }
   });
 }