Cleanup: get rid of the variadic parameter of ContextMgr.Push

Apparently this was an attempt at working around go's lack of default arguments,
but it's very unidiomatic and a bit confusing. Make it a normal parameter
instead, so all clients have to pass it explicitly.

pkg/gui/controllers/switch_to_diff_files_controller.go

@@ -91,7 +91,7 @@ func (self *SwitchToDiffFilesController) enter() error {
 		return err
 	}
 
-	self.c.Context().Push(commitFilesContext)
+	self.c.Context().Push(commitFilesContext, types.OnFocusOpts{})
 	return nil
 }