Fix relative symlinks when using --follow-symlinks

[kkebo]

fixes #836

Changes:

• Add a relative symlink to FileIteratorTests
  • This causes test failures if the following fixes are not applied
• Fix relative symlinks when using --follow-symlinks
• Eliminate duplicated files with standardizedFileURL when comparing them in FileIterator.next()

[kkebo]

It seemed that testFollowSymlinks failed on macOS. I'll investigate it.

https://ci.swift.org/job/swift-syntax-PR-macOS/4601/console

Test Case '-[SwiftFormatTests.FileIteratorTests testFollowSymlinks]' started.
/Users/ec2-user/jenkins/workspace/swift-syntax-PR-macOS/branch-main/swift-format/Tests/SwiftFormatTests/Utilities/FileIteratorTests.swift:37: error: -[SwiftFormatTests.FileIteratorTests testFollowSymlinks] : XCTAssertEqual failed: ("4") is not equal to ("3")
Test Case '-[SwiftFormatTests.FileIteratorTests testFollowSymlinks]' failed (0.699 seconds).

[kkebo]

The above error did not occur on macOS Sequoia. So I will prepare macOS Ventura. However, it will take time.

[kkebo]

I have successfully reproduced the issue on macOS 13.6.9 on EC2.

When the test failed, the local variable seen looked like this:

[
    "/private/var/folders/5r/59x37fyj13l2hjhq53g287_80000gn/T/TemporaryItems/NSIRD_xctest_Jjw9up/project/real2.swift",
    "/private/var/folders/5r/59x37fyj13l2hjhq53g287_80000gn/T/TemporaryItems/NSIRD_xctest_Jjw9up/project/.hidden.swift",
    "/var/folders/5r/59x37fyj13l2hjhq53g287_80000gn/T/TemporaryItems/NSIRD_xctest_Jjw9up/project/.hidden.swift",
    "/private/var/folders/5r/59x37fyj13l2hjhq53g287_80000gn/T/TemporaryItems/NSIRD_xctest_Jjw9up/project/real1.swift"
]

So it seems that this test failure occurs if a relative symbolic link is linked to a file that its path contains symbolic links.

[kkebo]

Fixed: ef73806

[ahoppen]

Thanks for the PR and tracking down the failure, @kkebo. Looks good to me, just two very minor comments.