Fix: Resolve the issue where the directory itself being a symlink does not work.

[hanxi]

Summary

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x] I've updated the documentation accordingly.

Fixed #2646

I test https://github.com/encode/starlette/security/advisories/GHSA-v5gw-mw7f-84px case. test result is 404 Not Found.

[Kludex]

A test case is needed here.

[hanxi]

Okay, I'll add the test cases.

[hanxi]

done.

[Kludex]

Thanks!