CleanOptions.Dir differs from -d flag in git clean

[msuozzo]

Per the docs:

Normally, when no is specified, git clean will not recurse into untracked directories to avoid removing too much. Specify -d to have it recurse into such directories as well.

But from the current implementation:

	for _, fi := range files {
		// <snip>
		if fi.IsDir() {
			if !opts.Dir {
				continue
			}
			// <snip>
		}
		// <snip>
	}

This means when Dir is false, Worktree.Clean won't recurse into any subdirs, not just the "untracked" ones. I suspect this is because go-git appears to lack a notion of "untracked directory" as Status operates solely on files.

This is a pretty major disconnect in the semantics between git and go-git and I think should be considered an implementation bug.