git-gui: allow closing console window with Escape

This gives users a quick shortcut to close the window. But since the
window can also show commands in progress, closing the window on Escape
can give the perception that the command has been cancelled even though
it hasn't been. So, only enable this binding when the command is done.

Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>

lib/console.tcl

@@ -203,6 +203,8 @@ method done {ok} {
 			focus $w.ok
 		}
 	}
+
+	bind $w <Key-Escape> "destroy $w;break"
 }
 
 method _sb_set {sb orient first last} {