vfs: fix open file renaming on drive when using --vfs-cache-mode writes

Before this change, when uploading files from the VFS cache which were
pending a rename, rclone would use the new path of the object when
specifiying the destination remote.  This didn't cause a problem with
most backends as the subsequent rename did nothing, however with the
drive backend, since it updates objects, the incorrect Remote was
embedded in the object.  This caused the rename to apparently succeed
but the object be at the wrong location.

The fix for this was to make sure we upload to the path stored in the
object if available.

This problem was spotted by the new rename tests for the VFS layer.

vfs/read_write.go

@@ -284,7 +284,12 @@ func (fh *RWFileHandle) flushWrites(closeFile bool) error {
 			return err
 		}
 
-		o, err := copyObj(fh.d.vfs.f, fh.file.getObject(), fh.file.Path(), cacheObj)
+		objPath := fh.file.Path()
+		objOld := fh.file.getObject()
+		if objOld != nil {
+			objPath = objOld.Remote() // use the path of the actual object if available
+		}
+		o, err := copyObj(fh.d.vfs.f, objOld, objPath, cacheObj)
 		if err != nil {
 			err = errors.Wrap(err, "failed to transfer file from cache to remote")
 			fs.Errorf(fh.logPrefix(), "%v", err)