drupal - Checking out a project that is using Git subtree -

cannot seem find answer basic process anywhere on web.

i've created new project @ folder my-project , created local git repo , linked remote repo origin.

inside folder my-project, i've used git subtree add add subfolder www clone of drupal git repo.

i've applied minor changes original drupal code.

i've committed/pushed remote repo.

all of working fine , can merge new versions drupal repo local repo push remote repo.

the problem when need clone new machine.

i start cloning repo add files in folder www well.

i run git remote add add drupal repo, , try run git subtree add or merge or pull fail because don't expect www folder there.

what correct command run re-link www folder drupal repo while keeping minor changes?

these errors when try different subtree commands:

1. add

   git subtree add --prefix=www drupal 8.3.x

message: prefix 'www' exists.

2. pull

   git subtree pull --prefix=www drupal 8.3.x

message: working tree has modifications. cannot add.

3. merge

   git subtree merge --prefix=www drupal 8.3.x

message:

fatal: ambiguous argument 'drupal': unknown revision or path not in working tree. use '--' separate paths revisions, this: 'git <command> [<revision>...] -- [<file>...]'