`URL(fileURLWithPath:relativeTo:)` regression on Linux

[nicklockwood]

The following code works as expected on macOS and earlier Linux versions but is broken on the latest nightly:

let path = "../images/foo.png"
let baseURL = URL(fileURLWithPath: "/docs/src/")
let result = URL(fileURLWithPath: path, relativeTo: baseURL)
print(result.path)

Expected:

/docs/images/foo.png

Actual:

/docs/src/images/foo.png

(i.e. the leading ../ in the path is ignored)