This is useful when you want to create a worktree from let's say master
branch and start a new branch. This basically reproduce git-worktree -b
options.
`add` behaves the same as before. `rm` removes worktrees.
`git worktree remove` on the gst-build worktree will delete the
subproject worktrees inside it, but will not remove the references to
them in the main repository's subproject worktrees. The `rm` command
will.
