Hard-code "enter" for editable prompts

Rebinding the universal.confirm keybinding currently doesn't make sense, because
the rebound key would also be used for editable prompts, which means you would
only be able to bind it to a ctrl key (not "y", which is desirable for some
people), and also it would allow you to enter a line feed in a branch name.

Fix this by always using enter for editable prompts.

pkg/gui/controllers/prompt_controller.go

@@ -27,7 +27,7 @@ func NewPromptController(
 func (self *PromptController) GetKeybindings(opts types.KeybindingsOpts) []*types.Binding {
 	bindings := []*types.Binding{
 		{
-			Key:             opts.GetKey(opts.Config.Universal.Confirm),
+			Key:             gocui.KeyEnter,
 			Handler:         func() error { return self.context().State.OnConfirm() },
 			Description:     self.c.Tr.Confirm,
 			DisplayOnScreen: true,