combine: fix CopyDirMetadata error on upstream root

Before this change, operations.CopyDirMetadata would fail with: `internal error:
expecting directory string from combine root '' to have SetMetadata method:
optional feature not implemented` if the dst was the root directory of a combine
upstream. This is because combine was returning a *fs.Dir, which does not
satisfy the fs.SetMetadataer interface.

While it is true that combine cannot set metadata on the root of an upstream
(see also #7652), this should not be considered an error that causes sync to do
high-level retries, abort without doing deletes, etc.

This change addresses the issue by creating a new type of DirWrapper that is
allowed to fail silently, for exceptional cases such as this where certain
special directories have more limited abilities than what the Fs usually
supports.

It is possible that other similar wrapping backends (Union?) may need this same
fix.

backend/combine/combine.go

@@ -813,7 +813,7 @@ func (f *Fs) List(ctx context.Context, dir string) (entries fs.DirEntries, err e
 	if f.root == "" && dir == "" {
 		entries = make(fs.DirEntries, 0, len(f.upstreams))
 		for combineDir := range f.upstreams {
-			d := fs.NewDir(combineDir, f.when)
+			d := fs.NewLimitedDirWrapper(combineDir, fs.NewDir(combineDir, f.when))
 			entries = append(entries, d)
 		}
 		return entries, nil
@@ -1002,7 +1002,7 @@ func (f *Fs) DirSetModTime(ctx context.Context, dir string, modTime time.Time) e
 		return err
 	}
 	if uDir == "" {
-		fs.Debugf(dir, "can't set modtime on upstream root. skipping.")
+		fs.Debugf(dir, "Can't set modtime on upstream root. skipping.")
 		return nil
 	}
 	if do := u.f.Features().DirSetModTime; do != nil {