drive: don't delete files with multiple parents to avoid data loss

Rclone can't safely delete files with multiple parents without
PATCHing the parents list. This can be done, but since multiple
parents are going away to be replaced by drive shortcuts we return an
error for now.

See #4013

backend/drive/drive.go

@@ -551,6 +551,7 @@ type baseObject struct {
 	modifiedDate string // RFC3339 time it was last modified
 	mimeType     string // The object MIME type
 	bytes        int64  // size of the object
+	parents      int    // number of parents
 }
 type documentObject struct {
 	baseObject
@@ -1176,6 +1177,7 @@ func (f *Fs) newBaseObject(remote string, info *drive.File) baseObject {
 		modifiedDate: modifiedDate,
 		mimeType:     info.MimeType,
 		bytes:        size,
+		parents:      len(info.Parents),
 	}
 }
 
@@ -2989,6 +2991,9 @@ func (o *linkObject) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo
 
 // Remove an object
 func (o *baseObject) Remove(ctx context.Context) error {
+	if o.parents > 1 {
+		return errors.New("can't delete safely - has multiple parents")
+	}
 	var err error
 	err = o.fs.pacer.Call(func() (bool, error) {
 		if o.fs.opt.UseTrash {