wtadd not using separate branch name argument

[msorens]

The advertised interface for wtadd is wtadd [-vh] WORKTREE_NAME [BRANCH_NAME]. However, the BRANCH_NAME argument is ignored; both the working directory and the branch use WORKTREE_NAME.

Should either update the usage details or change the code to match current usage details.

BTW, thanks for a great article on worktrees and this accompanying toolset!!