Show hint about hunk staging mode being the default now, and how to switch to line mode

It is shown the first time the user enters either staging or patch building.

pkg/gui/controllers/files_controller.go

@@ -554,6 +554,8 @@ func (self *FilesController) EnterFile(opts types.OnFocusOpts) error {
 
 	context := lo.Ternary(opts.ClickedWindowName == "secondary", self.c.Contexts().StagingSecondary, self.c.Contexts().Staging)
 	self.c.Context().Push(context, opts)
+	self.c.Helpers().PatchBuilding.ShowHunkStagingHint()
+
 	return nil
 }