envbuilder: fetch upstream changes from repo if local copy is not dirty

Is it possible to disable the /workspace cache of a coder build with envbuilder.
Even after restart work spaces it caches to old commit

```
#1: 📦 Cloning https://bitbucket.org/lorem/repo.git to /workspaces/ipsum..
#1: 👤 Using no authentication!
#1: 📦 The repository already exists! [672.333µs]
```

首先跟踪 repository 复用和 /workspace 缓存处理，查明它们如何在报告的日志中产生“The repository already exists!”。检查如何检测本地更改，并将完成条件定义为：现有副本干净时获取 upstream commit，而副本不干净时保留本地工作。

由索引模型根据 Issue 内容生成。