- When a user goes opens a symlink file in Forgejo, the file would be
rendered with the path of the symlink as content.
- Add a button that is shown when the user opens a *valid* symlink file,
which means that the symlink must have an valid path to an existent
file and after 999 follows isn't a symlink anymore.
- Return the relative path from the `FollowLink` functions, because Git
really doesn't want to tell where an file is located based on the blob ID.
- Adds integration tests.
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