Add `title` to PR file tree items (#22918)

Previously, a file/directory name was simply cut when it was too long.
Now, we display the browser-native tooltip (`title`) instead, so you can
still see it when hovering over it.
In this case, we don't use the normal `tippy` tooltips for three
reasons:
1. Vue components are not included in the global tooltip initialization
2. Vue components would need to initialize their tooltips themselves
whenever their content is changed
3. The tooltips are shown too long under the default configuration (the
tooltip one element above is still shown when hovering on the element
below)

Fixes #22915

## Appearance


![image](https://user-images.githubusercontent.com/51889757/219049642-43668a38-0e86-42bf-a1d0-3742c4dc7fd9.png)

## Room for future improvement

We could think about displaying the whole file path in the title, not
just its name.
This is not done at the moment:

![image](https://user-images.githubusercontent.com/51889757/219050689-1e6e3d57-f2bf-48be-8553-415e744a6e10.png)

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Co-authored-by: techknowlogick <techknowlogick@gitea.io>

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