Some bugs around "untracked cache" feature have been fixed.

* nd/fix-untracked-cache-invalidation:
  dir.c: ignore paths containing .git when invalidating untracked cache
  dir.c: stop ignoring opendir() error in open_cached_dir()
  dir.c: fix missing dir invalidation in untracked code
  dir.c: avoid stat() in valid_cached_dir()
  status: add a failing test showing a core.untrackedCache bug