worktree: add ability to create worktree with pre-existing branch

Currently, we always create a new branch after the new worktree's name
when creating a worktree. In some workflows, though, the caller may want
to check out an already existing reference instead of creating a new
one, which is impossible to do right now.

Add a new option `ref` to the options structure for adding worktrees. In
case it is set, a branch and not already checked out by another
worktree, we will re-use this reference instead of creating a new one.

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