Fix PR file tree folders no longer collapsing (#22864)

Fix PR file tree folders no longer collapsing (#22864)

Collapsing folders currently just throws a console error

```
index.js?v=1.19.0~dev-403-gb6b8feb3d:10 TypeError: this.$set is not a function
    at Proxy.handleClick (index.js?v=1.19.0~dev-403-gb6b8feb3d:58:7159)
    at index.js?v=1.19.0~dev-403-gb6b8feb3d:58:6466
    at index.js?v=1.19.0~dev-403-gb6b8feb3d:10:93922
    at ce (index.js?v=1.19.0~dev-403-gb6b8feb3d:10:1472)
    at Q (index.js?v=1.19.0~dev-403-gb6b8feb3d:10:1567)
    at HTMLDivElement.$e (index.js?v=1.19.0~dev-403-gb6b8feb3d:10:79198)
```

This PR fixes this and allows folders to be collapsed again.

Also:
- better cursor interaction with folders
- added some color to the diff detail stats
- remove green link color from all the file names

Screenshots:

![image](https://user-images.githubusercontent.com/9765622/218269712-2f3dda55-6d70-407f-8d34-2a5d9c8df548.png)

![image](https://user-images.githubusercontent.com/9765622/218269714-6ce8a954-daea-4ed6-9eea-8b2323db4d8f.png)

---------

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>