lib/fs: Handle deduplicated files on NTFS (fixes #1845)

These files always have the symlink bit set, because they are reparse
points. Nonetheless they are not symlinks, and Lstat reports a size for
them. We use this fact to disambiguate, and hope fervently that nothing
else matches this description so it comes back to bite us...

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4622

lib/fs/fsfileinfo_windows.go

@@ -0,0 +1,21 @@
+// Copyright (C) 2017 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package fs
+
+import (
+	"os"
+)
+
+func (e fsFileInfo) Mode() FileMode {
+	m := e.FileInfo.Mode()
+	if m&os.ModeSymlink != 0 && e.Size() > 0 {
+		// "Symlinks" with nonzero size are in fact "hard" links, such as
+		// NTFS deduped files. Remove the symlink bit.
+		m &^= os.ModeSymlink
+	}
+	return FileMode(m)
+}