Avoid calling stat twice for each item returned by NSURLDirectoryEnumerator

fts_read calls stat internally to determine the type of each file that is visited. If URL(fileURLWithPath:) is called
with a path that doesn't end with a slash, it calls stat to determine whether the path represents a file or a
directory. We know from the FTSENT's fts_info field whether the entry represents a file or a directory, so we can
instead call URL(fileURLWithPath:isDirectory:) and avoid the second stat call.

This reduces the runtime of FileManager.enumerator(at:includingPropertiesForKeys:options:errorHandler:) by around 40%.

Author: bdash

Foundation/FileManager+POSIX.swift

@@ -1099,13 +1099,13 @@ internal func _contentsEqual(atPath path1: String, andPath path2: String) -> Boo
                                 fts_set(_stream, _current, FTS_SKIP)
                             }
                             if showFile {
-                                 return URL(fileURLWithPath: filename)
+                                 return URL(fileURLWithPath: filename, isDirectory: true)
                             }
 
                         case FTS_DEFAULT, FTS_F, FTS_NSOK, FTS_SL, FTS_SLNONE:
                             let (showFile, _) = match(filename: filename, to: _options, isDir: false)
                             if showFile {
-                                return URL(fileURLWithPath: filename)
+                                return URL(fileURLWithPath: filename, isDirectory: false)
                             }
                         case FTS_DNR, FTS_ERR, FTS_NS:
                             let keepGoing: Bool