A relative FORGIT_WORKTREE_ADD_DIR is resolved against the root of the
main worktree on every invocation, so a single setting such as
`.worktrees` is valid in every repository. Absolute paths keep their
meaning.
The variable previously only took a full path, so users resolved the
repository root in their shell startup files, which runs git outside a
repository on every new shell and freezes the path for the whole
session.