main Fix support for FileIterator when working directory is /

Added an additional check that tests to see if the working directory is root.
swiftlang#862

Author: macshome

Sources/SwiftFormat/Utilities/FileIterator.swift

@@ -145,7 +145,7 @@ public struct FileIterator: Sequence, IteratorProtocol {
         // be displayed as relative paths. Otherwise, they will still be displayed as absolute
         // paths.
         let relativePath =
-          path.hasPrefix(workingDirectory.path)
+          path.hasPrefix(workingDirectory.path) && path.first != "/"
           ? String(path.dropFirst(workingDirectory.path.count + 1))
           : path
         output =