Version history for issues created prior to using Github Backup

When using Github Backup App on an existing project with existing issues, version history of these existing issues are not displayed in the meta table on Github. Version history is only displayed in the meta table for issues that have been added once Github Backup is in use.

It would be useful to a user of Github Backup App to see version history for issues created prior to using the app. This could be done during the installation process.

首先跟踪安装过程以及构建 issue 元数据表和版本历史记录的代码。确定如何在安装过程中处理现有 issue，然后验证它们之前的版本历史记录是否显示在 GitHub 元数据表中，同时不改变对新备份 issue 的行为。

由索引模型根据 Issue 内容生成。