[He has no idea what that first response means, unless Luke's universe is another without Earth. It's entirely possible. It also means the joke fell completely flat. Oh well.
Garrus re-reads Luke's file as he heads over. Telekinesis, telepathy. That means Luke will know he's there, can stop the bullet with that, too, but he's still a little not sure about just taking a shot when he's ready. Yes, Luke said to. But just in case, once Garrus is in position, he calls out:]
In five!
[Maybe it's far more warning than Luke needs, but he's not going to risk it. Luke is no enemy. And then, five seconds later, Garrus takes the shot.]
Action
Garrus re-reads Luke's file as he heads over. Telekinesis, telepathy. That means Luke will know he's there, can stop the bullet with that, too, but he's still a little not sure about just taking a shot when he's ready. Yes, Luke said to. But just in case, once Garrus is in position, he calls out:]
In five!
[Maybe it's far more warning than Luke needs, but he's not going to risk it. Luke is no enemy. And then, five seconds later, Garrus takes the shot.]